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unknown-crash in S_format_hexfp #16942

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p5pRT opened this issue Apr 9, 2019 · 12 comments
Closed

unknown-crash in S_format_hexfp #16942

p5pRT opened this issue Apr 9, 2019 · 12 comments

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@p5pRT p5pRT commented Apr 9, 2019

Migrated from rt.perl.org#134008 (status was 'pending release')

Searchable as RT134008$

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@p5pRT p5pRT commented Apr 9, 2019

From @dur-randir

Created by @dur-randir

While fuzzing perl v5.29.9-63-g2496d8f3f7 built with afl and run
under libdislocator, I found the following progam

printf "%.*a", 1.84466502487128e+19,0

to cause unknown-crash write diagnostics by ASAN. GDB stack trace is following

S_format_hexfp (buf=0x555555b75b80 "0x0.", '0' <repeats 196 times>...,
bufsize=69, c=97 'a', nv=0, fv=0, has_precis=false,
precis=93824996751360, width=0,
  alt=false, plus=0 '\000', left=false, fill=false) at sv.c​:11792
11792 *p++ = '0';
(gdb) bt
#0 S_format_hexfp (buf=0x555555b75b80 "0x0.", '0' <repeats 196
times>..., bufsize=69, c=97 'a', nv=0, fv=0, has_precis=false,
precis=93824996751360,
  width=0, alt=false, plus=0 '\000', left=false, fill=false) at sv.c​:11792
#1 0x00005555557d6c1c in Perl_sv_vcatpvfn_flags (sv=0x555555b4e740,
pat=0x555555b75b10 "%.*a", patlen=4, args=0x0, svargs=0x555555b50db0,
sv_count=2,
  maybe_tainted=0x7fffffffdd97, flags=0) at sv.c​:13093
#2 0x00005555557cff0e in Perl_sv_vsetpvfn (sv=0x555555b4e740,
pat=0x555555b75b10 "%.*a", patlen=4, args=0x0, svargs=0x555555b50db0,
sv_count=2,
  maybe_tainted=0x7fffffffdd97) at sv.c​:10977
#3 0x000055555587eb55 in Perl_do_sprintf (sv=0x555555b4e740, len=3,
sarg=0x555555b50da8) at doop.c​:734
#4 0x000055555585cad0 in Perl_pp_prtf () at pp_sys.c​:1628
#5 0x000055555570b895 in Perl_runops_debug () at dump.c​:2537
#6 0x00005555555ed560 in S_run_body (oldscope=1) at perl.c​:2716
#7 0x00005555555ecade in perl_run (my_perl=0x555555b4c260) at perl.c​:2639
#8 0x00005555555a114e in main (argc=3, argv=0x7fffffffe1a8,
env=0x7fffffffe1c8) at perlmain.c​:127

This is a regression between 5.26 and 5.28, bisect points to​:

commit 50a7222 (HEAD, refs/bisect/bad)
Author​: David Mitchell <davem@​iabyn.com>
Date​: Tue May 30 13​:45​:35 2017 +0100

  Perl_sv_vcatpvfn_flags​: width/precis arg wrap

  When the width or precision is specified via an argument rather than
  literally, check whether the value wraps.

Perl Info

Flags:
    category=core
    severity=high

Site configuration information for perl 5.29.9:

Configured by dur-randir at Wed Feb 27 14:51:01 MSK 2019.

Summary of my perl5 (revision 5 version 29 subversion 9) configuration:
  Commit id: c1e47bad34ce1d9c84ed57c9b8978bcbd5a02e98
  Platform:
    osname=darwin
    osvers=13.4.0
    archname=darwin-thread-multi-2level
    uname='darwin isengard.local 13.4.0 darwin kernel version 13.4.0:
mon jan 11 18:17:34 pst 2016; root:xnu-2422.115.15~1release_x86_64
x86_64 '
    config_args='-de -Dusedevel -DDEBUGGING -Dusethreads'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='cc'
    ccflags ='-fno-common -DPERL_DARWIN -mmacosx-version-min=10.9
-DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector
-I/usr/local/include -DPERL_USE_SAFE_PUTENV'
    optimize='-O3 -g'
    cppflags='-fno-common -DPERL_DARWIN -mmacosx-version-min=10.9
-DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector
-I/usr/local/include'
    ccversion=''
    gccversion='4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.56)'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='cc'
    ldflags =' -mmacosx-version-min=10.9 -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/6.0/lib
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib
/usr/lib
    libs=-lpthread -lgdbm -ldbm -ldl -lm -lutil -lc
    perllibs=-lpthread -ldl -lm -lutil -lc
    libc=
    so=dylib
    useshrplib=false
    libperl=libperl.a
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=bundle
    d_dlsymun=undef
    ccdlflags=' '
    cccdlflags=' '
    lddlflags=' -mmacosx-version-min=10.9 -bundle -undefined
dynamic_lookup -L/usr/local/lib -fstack-protector'



@INC for perl 5.29.9:
    lib
    /usr/local/lib/perl5/site_perl/5.29.9/darwin-thread-multi-2level
    /usr/local/lib/perl5/site_perl/5.29.9
    /usr/local/lib/perl5/5.29.9/darwin-thread-multi-2level
    /usr/local/lib/perl5/5.29.9


Environment for perl 5.29.9:
    DYLD_LIBRARY_PATH (unset)
    HOME=/Users/dur-randir
    LANG=en_US.UTF-8
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/Users/dur-randir/perlbrew/bin:/Users/dur-randir/perlbrew/perls/perl-5.22.1/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/texbin
    PERLBREW_HOME=/Users/dur-randir/.perlbrew
    PERLBREW_MANPATH=/Users/dur-randir/perlbrew/perls/perl-5.22.1/man
    PERLBREW_PATH=/Users/dur-randir/perlbrew/bin:/Users/dur-randir/perlbrew/perls/perl-5.22.1/bin
    PERLBREW_PERL=perl-5.22.1
    PERLBREW_ROOT=/Users/dur-randir/perlbrew
    PERLBREW_SHELLRC_VERSION=0.84
    PERLBREW_VERSION=0.84
    PERL_BADLANG (unset)
    SHELL=/usr/local/bin/zsh

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@p5pRT p5pRT commented Apr 9, 2019

From @hvds

On Mon, 08 Apr 2019 23​:40​:01 -0700, randir wrote​:

While fuzzing perl v5.29.9-63-g2496d8f3f7 built with afl and run
under libdislocator, I found the following progam

printf "%.*a", 1.84466502487128e+19,0

to cause unknown-crash write diagnostics by ASAN. GDB stack trace is
following

We don't need wraparound, it's enough to provide the negative precision directly, with various outcomes​:
  % ./miniperl -e 'sprintf "%.*a", -0x18800,0'
  Segmentation fault (core dumped)
  % ./miniperl -e 'sprintf "%.*a", -0x18400,0'
  *** Error in `./miniperl'​: realloc()​: invalid next size​: 0x000000000167cb10 ***
  Aborted (core dumped)
  % /miniperl -e 'sprintf "%.*a", -0x40,0'
  panic​: snprintf buffer overflow at -e line 1.
  %

I'm not aware of any intended meaning for a negative precision, but in the sv_vcatpvfn_flags() code we set 'precis' to the absolute value while leaving 'has_precis' false. Most of the rest then ignores precis unless has_precis, but this code in S_format_hexfp() uses it, as does the tail code in sv_vcatpvfn_flags() after the comment​:
  /* append esignbuf, filler, zeros, eptr and dotstr to sv */

The trivial patch below protects from this core dump and passes tests, but I think we need to consider the potential impact here carefully, as well as clean up and clarify the code.

Hugo

Inline Patch
diff --git a/sv.c b/sv.c
index b6d9123..fe26d13 100644
--- a/sv.c
+++ b/sv.c
@@ -12215,7 +12215,10 @@ Perl_sv_vcatpvfn_flags(pTHX_ SV *const sv, const char *const pat, const STRLEN p
                                           : (arg_missing = TRUE, (SV*)NULL);
                     }
                     precis = S_sprintf_arg_num_val(aTHX_ args, i, sv, &neg);
-                    has_precis = !neg;
+                    if (neg)
+                        precis = 0;
+                    else
+                        has_precis = true;
                 }
            }
            else {
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@p5pRT p5pRT commented Apr 9, 2019

The RT System itself - Status changed from 'new' to 'open'

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@p5pRT p5pRT commented Apr 9, 2019

From @hvds

On Tue, 09 Apr 2019 04​:47​:13 -0700, hv wrote​:

the tail code in sv_vcatpvfn_flags() after the comment​:
/* append esignbuf, filler, zeros, eptr and dotstr to sv */

It turns out the only use here is if 'zeros' is true, which is only set in a case where has_precis is true.

So as far as I can tell, the only problem here is for hexfp, and the payload is to overwrite a number of bytes beyond the PL_efloatbuf buffer with '0' characters, the number being controlled by a sprintf argument.

As such I think the risk here is low - if a user can provide the sprintf pattern, you've probably already lost, and likelihood of a '%.*a' pattern with user-supplied arguments seems remote. Nothing in the test suite uses negative precision except specific sprintf{,2} tests asserting it should be ignored (in the absence of which I'd be tempted to make it fatal instead).

So I propose that we commit the attached belt-and-braces patch, open the ticket, and consider this for backporting. I'd welcome other opinions.

Hugo

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@p5pRT p5pRT commented Apr 9, 2019

From @hvds

0001-134008-More-carefully-ignore-negative-precision-in-s.patch
From 6a3439c5a14d6026d16995e539c44f58e52a102a Mon Sep 17 00:00:00 2001
From: Hugo van der Sanden <hv@crypt.org>
Date: Tue, 9 Apr 2019 14:27:41 +0100
Subject: [PATCH] [#134008] More carefully ignore negative precision in sprintf

Check has_precis more consistently; ensure precis is left as 0 if provided
as a negative number.
---
 sv.c            | 7 +++++--
 t/op/sprintf2.t | 3 +++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/sv.c b/sv.c
index b6d9123..611a9f4 100644
--- a/sv.c
+++ b/sv.c
@@ -11758,11 +11758,11 @@ S_format_hexfp(pTHX_ char * const buf, const STRLEN bufsize, const char c,
     else {
         *p++ = '0';
         exponent = 0;
-        zerotail = precis;
+        zerotail = has_precis ? precis : 0;
     }
 
     /* The radix is always output if precis, or if alt. */
-    if (precis > 0 || alt) {
+    if ((has_precis && precis > 0) || alt) {
       hexradix = TRUE;
     }
 
@@ -12216,6 +12216,9 @@ Perl_sv_vcatpvfn_flags(pTHX_ SV *const sv, const char *const pat, const STRLEN p
                     }
                     precis = S_sprintf_arg_num_val(aTHX_ args, i, sv, &neg);
                     has_precis = !neg;
+                    /* ignore negative precision */
+                    if (!has_precis)
+                        precis = 0;
                 }
 	    }
 	    else {
diff --git a/t/op/sprintf2.t b/t/op/sprintf2.t
index 3f4c126..891eb05 100644
--- a/t/op/sprintf2.t
+++ b/t/op/sprintf2.t
@@ -830,6 +830,9 @@ SKIP: {
     # [rt.perl.org #128889]
     is(sprintf("%.*a", -1, 1.03125), "0x1.08p+0", "[rt.perl.org #128889]");
 
+    # [rt.perl.org #134008]
+    is(sprintf("%.*a", -99999, 1.03125), "0x1.08p+0", "[rt.perl.org #134008]");
+
     # [rt.perl.org #128890]
     is(sprintf("%a", 0x1.18p+0), "0x1.18p+0");
     is(sprintf("%.1a", 0x1.08p+0), "0x1.0p+0");
-- 
2.10.2

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@p5pRT p5pRT commented Apr 16, 2019

From @tonycoz

On Mon, 08 Apr 2019 23​:40​:01 -0700, randir wrote​:

This is a bug report for perl from sergey.aleynikov@​gmail.com,
generated with the help of perlbug 1.41 running under perl 5.29.9.

-----------------------------------------------------------------
[Please describe your issue here]

While fuzzing perl v5.29.9-63-g2496d8f3f7 built with afl and run
under libdislocator, I found the following progam

printf "%.*a", 1.84466502487128e+19,0

to cause unknown-crash write diagnostics by ASAN. GDB stack trace is
following

S_format_hexfp (buf=0x555555b75b80 "0x0.", '0' <repeats 196 times>...,
bufsize=69, c=97 'a', nv=0, fv=0, has_precis=false,
precis=93824996751360, width=0,
alt=false, plus=0 '\000', left=false, fill=false) at sv.c​:11792
11792 *p++ = '0';
(gdb) bt
#0 S_format_hexfp (buf=0x555555b75b80 "0x0.", '0' <repeats 196
times> ..., bufsize=69, c=97 'a', nv=0, fv=0, has_precis=false,
precis=93824996751360,
width=0, alt=false, plus=0 '\000', left=false, fill=false) at
sv.c​:11792
#1 0x00005555557d6c1c in Perl_sv_vcatpvfn_flags (sv=0x555555b4e740,
pat=0x555555b75b10 "%.*a", patlen=4, args=0x0, svargs=0x555555b50db0,
sv_count=2,
maybe_tainted=0x7fffffffdd97, flags=0) at sv.c​:13093
#2 0x00005555557cff0e in Perl_sv_vsetpvfn (sv=0x555555b4e740,
pat=0x555555b75b10 "%.*a", patlen=4, args=0x0, svargs=0x555555b50db0,
sv_count=2,
maybe_tainted=0x7fffffffdd97) at sv.c​:10977
#3 0x000055555587eb55 in Perl_do_sprintf (sv=0x555555b4e740, len=3,
sarg=0x555555b50da8) at doop.c​:734
#4 0x000055555585cad0 in Perl_pp_prtf () at pp_sys.c​:1628
#5 0x000055555570b895 in Perl_runops_debug () at dump.c​:2537
#6 0x00005555555ed560 in S_run_body (oldscope=1) at perl.c​:2716
#7 0x00005555555ecade in perl_run (my_perl=0x555555b4c260) at
perl.c​:2639
#8 0x00005555555a114e in main (argc=3, argv=0x7fffffffe1a8,
env=0x7fffffffe1c8) at perlmain.c​:127

This is a regression between 5.26 and 5.28, bisect points to​:

commit 50a7222 (HEAD,
refs/bisect/bad)
Author​: David Mitchell <davem@​iabyn.com>
Date​: Tue May 30 13​:45​:35 2017 +0100

Perl_sv_vcatpvfn_flags​: width/precis arg wrap

When the width or precision is specified via an argument rather than
literally, check whether the value wraps.

The problem in this case is that S_format_hexfp wasn't handling has_precis/precis
correctly.

I do think this is a security issue, though code that encounters it likely has other problems.

Tony

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@p5pRT p5pRT commented Apr 16, 2019

From @tonycoz

0001-perl-134008-correct-precision-handling-in-S_format_h.patch
From 48a6726d70db547ac5edaedba32d02ed028b9e70 Mon Sep 17 00:00:00 2001
From: Tony Cook <tony@develop-help.com>
Date: Tue, 16 Apr 2019 10:36:45 +1000
Subject: (perl #134008) correct precision handling in S_format_hexfp

A negative precision via .* is intended to be treated as if no
precision value was supplied at all, and the code in
sv_vcatpvfn_flags() handles that correctly, unfortunately
S_format_hexfp continued to use the precis value even when has_precis
is false and since precis isn't used in calculating the buffer size
when has_precis is false, this can lead to a buffer overflow.
---
 sv.c            | 4 ++--
 t/op/sprintf2.t | 9 +++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/sv.c b/sv.c
index 9b659e8c16..812ed65b1a 100644
--- a/sv.c
+++ b/sv.c
@@ -11759,11 +11759,11 @@ S_format_hexfp(pTHX_ char * const buf, const STRLEN bufsize, const char c,
     else {
         *p++ = '0';
         exponent = 0;
-        zerotail = precis;
+        zerotail = has_precis ? precis : 0;
     }
 
     /* The radix is always output if precis, or if alt. */
-    if (precis > 0 || alt) {
+    if ((has_precis && precis > 0) || alt) {
       hexradix = TRUE;
     }
 
diff --git a/t/op/sprintf2.t b/t/op/sprintf2.t
index 3f4c126c68..590273e8a2 100644
--- a/t/op/sprintf2.t
+++ b/t/op/sprintf2.t
@@ -1141,4 +1141,13 @@ foreach(
     is sprintf("%.0f", $_), sprintf("%-.0f", $_), "special-case %.0f on $_";
 }
 
+{
+    # 134008
+    # the large number in the test case from the ticket might be
+    # interpreted differently on 32-bit platforms, so use a literal
+    # negative number that reproduces the problem
+    fresh_perl_is('printf "%.*a", -100000,0', '0x0p+0',
+                  {}, 'negative precision ignored by format_hexfp');
+}
+
 done_testing();
-- 
2.11.0

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@p5pRT p5pRT commented Apr 16, 2019

From @tonycoz

On Tue, 09 Apr 2019 07​:01​:25 -0700, hv wrote​:

On Tue, 09 Apr 2019 04​:47​:13 -0700, hv wrote​:

the tail code in sv_vcatpvfn_flags() after the comment​:
/* append esignbuf, filler, zeros, eptr and dotstr to sv */

It turns out the only use here is if 'zeros' is true, which is only
set in a case where has_precis is true.

So as far as I can tell, the only problem here is for hexfp, and the
payload is to overwrite a number of bytes beyond the PL_efloatbuf
buffer with '0' characters, the number being controlled by a sprintf
argument.

As such I think the risk here is low - if a user can provide the
sprintf pattern, you've probably already lost, and likelihood of a
'%.*a' pattern with user-supplied arguments seems remote. Nothing in
the test suite uses negative precision except specific sprintf{,2}
tests asserting it should be ignored (in the absence of which I'd be
tempted to make it fatal instead).

So I propose that we commit the attached belt-and-braces patch, open
the ticket, and consider this for backporting. I'd welcome other
opinions.

Somehow I didn't see Hugo's response (and his patch).

An attacker doesn't need to supply a format string, they just need to be able to supply a negative precision, and it doesn't need to be large in magnitude.

I could see a reporting tool allowing specifying width/precision for fields, though perhaps not so much supporting %a formatting.

A case could be made that such a tool is buggy if it permits very large or negative precisions, but that's irrelevant as to whether a bug in perl becomes a security issue for such code.

The behaviour for negative precision comes from the C standard​:

  A negative precision argument is taken as if the precision were omitted.

which presumably is what the current implementation is intended to emulate (especially since PerlIO_printf() uses this code.

Tony

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@p5pRT p5pRT commented Apr 16, 2019

From @hvds

On Mon, 15 Apr 2019 18​:17​:44 -0700, tonyc wrote​:

On Tue, 09 Apr 2019 07​:01​:25 -0700, hv wrote​:

On Tue, 09 Apr 2019 04​:47​:13 -0700, hv wrote​:

the tail code in sv_vcatpvfn_flags() after the comment​:
/* append esignbuf, filler, zeros, eptr and dotstr to sv */

It turns out the only use here is if 'zeros' is true, which is only
set in a case where has_precis is true.

So as far as I can tell, the only problem here is for hexfp, and the
payload is to overwrite a number of bytes beyond the PL_efloatbuf
buffer with '0' characters, the number being controlled by a sprintf
argument.

As such I think the risk here is low - if a user can provide the
sprintf pattern, you've probably already lost, and likelihood of a
'%.*a' pattern with user-supplied arguments seems remote. Nothing in
the test suite uses negative precision except specific sprintf{,2}
tests asserting it should be ignored (in the absence of which I'd be
tempted to make it fatal instead).

So I propose that we commit the attached belt-and-braces patch, open
the ticket, and consider this for backporting. I'd welcome other
opinions.

Somehow I didn't see Hugo's response (and his patch).

An attacker doesn't need to supply a format string, they just need to
be able to supply a negative precision, and it doesn't need to be
large in magnitude.

I could see a reporting tool allowing specifying width/precision for
fields, though perhaps not so much supporting %a formatting.

A case could be made that such a tool is buggy if it permits very
large or negative precisions, but that's irrelevant as to whether a
bug in perl becomes a security issue for such code.

The behaviour for negative precision comes from the C standard​:

A negative precision argument is taken as if the precision were
omitted.

which presumably is what the current implementation is intended to
emulate (especially since PerlIO_printf() uses this code.

No problem with your analysis, I did request other opinions.

I'd recommend adding the extra change in sv_vcatpvfn_flags from my patch. Your test looks like the better one though.

Hugo

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@p5pRT p5pRT commented May 1, 2019

From @iabyn

On Tue, Apr 16, 2019 at 01​:58​:32AM -0700, Hugo van der Sanden via RT wrote​:

On Mon, 15 Apr 2019 18​:17​:44 -0700, tonyc wrote​:

On Tue, 09 Apr 2019 07​:01​:25 -0700, hv wrote​:

On Tue, 09 Apr 2019 04​:47​:13 -0700, hv wrote​:

the tail code in sv_vcatpvfn_flags() after the comment​:
/* append esignbuf, filler, zeros, eptr and dotstr to sv */

It turns out the only use here is if 'zeros' is true, which is only
set in a case where has_precis is true.

So as far as I can tell, the only problem here is for hexfp, and the
payload is to overwrite a number of bytes beyond the PL_efloatbuf
buffer with '0' characters, the number being controlled by a sprintf
argument.

As such I think the risk here is low - if a user can provide the
sprintf pattern, you've probably already lost, and likelihood of a
'%.*a' pattern with user-supplied arguments seems remote. Nothing in
the test suite uses negative precision except specific sprintf{,2}
tests asserting it should be ignored (in the absence of which I'd be
tempted to make it fatal instead).

So I propose that we commit the attached belt-and-braces patch, open
the ticket, and consider this for backporting. I'd welcome other
opinions.

Somehow I didn't see Hugo's response (and his patch).

An attacker doesn't need to supply a format string, they just need to
be able to supply a negative precision, and it doesn't need to be
large in magnitude.

I could see a reporting tool allowing specifying width/precision for
fields, though perhaps not so much supporting %a formatting.

A case could be made that such a tool is buggy if it permits very
large or negative precisions, but that's irrelevant as to whether a
bug in perl becomes a security issue for such code.

The behaviour for negative precision comes from the C standard​:

A negative precision argument is taken as if the precision were
omitted.

which presumably is what the current implementation is intended to
emulate (especially since PerlIO_printf() uses this code.

No problem with your analysis, I did request other opinions.

I'd recommend adding the extra change in sv_vcatpvfn_flags from my patch. Your test looks like the better one though.

I recommend we go with the hybrid as suggested by Hugo. I think we should
add it to blead, and I don't think we need to treat it as a serious
security issue. The string '%.*a' as part of a format appears nowhere on
cpan.

--
Overhead, without any fuss, the stars were going out.
  -- Arthur C Clarke

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@p5pRT p5pRT commented Jun 3, 2019

From @tonycoz

On Wed, 01 May 2019 05​:36​:38 -0700, davem wrote​:

On Tue, Apr 16, 2019 at 01​:58​:32AM -0700, Hugo van der Sanden via RT
wrote​:

On Mon, 15 Apr 2019 18​:17​:44 -0700, tonyc wrote​:

On Tue, 09 Apr 2019 07​:01​:25 -0700, hv wrote​:

On Tue, 09 Apr 2019 04​:47​:13 -0700, hv wrote​:

the tail code in sv_vcatpvfn_flags() after the comment​:
/* append esignbuf, filler, zeros, eptr and dotstr to sv */

It turns out the only use here is if 'zeros' is true, which is
only
set in a case where has_precis is true.

So as far as I can tell, the only problem here is for hexfp, and
the
payload is to overwrite a number of bytes beyond the PL_efloatbuf
buffer with '0' characters, the number being controlled by a
sprintf
argument.

As such I think the risk here is low - if a user can provide the
sprintf pattern, you've probably already lost, and likelihood of
a
'%.*a' pattern with user-supplied arguments seems remote. Nothing
in
the test suite uses negative precision except specific
sprintf{,2}
tests asserting it should be ignored (in the absence of which I'd
be
tempted to make it fatal instead).

So I propose that we commit the attached belt-and-braces patch,
open
the ticket, and consider this for backporting. I'd welcome other
opinions.

Somehow I didn't see Hugo's response (and his patch).

An attacker doesn't need to supply a format string, they just need
to
be able to supply a negative precision, and it doesn't need to be
large in magnitude.

I could see a reporting tool allowing specifying width/precision
for
fields, though perhaps not so much supporting %a formatting.

A case could be made that such a tool is buggy if it permits very
large or negative precisions, but that's irrelevant as to whether a
bug in perl becomes a security issue for such code.

The behaviour for negative precision comes from the C standard​:

A negative precision argument is taken as if the precision were
omitted.

which presumably is what the current implementation is intended to
emulate (especially since PerlIO_printf() uses this code.

No problem with your analysis, I did request other opinions.

I'd recommend adding the extra change in sv_vcatpvfn_flags from my
patch. Your test looks like the better one though.

I recommend we go with the hybrid as suggested by Hugo. I think we
should
add it to blead, and I don't think we need to treat it as a serious
security issue. The string '%.*a' as part of a format appears nowhere
on
cpan.

Done in b0f5b1d, 9dfe0a3.

Tony

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@p5pRT p5pRT commented Jun 3, 2019

@tonycoz - Status changed from 'open' to 'pending release'

@p5pRT p5pRT closed this Jun 3, 2019
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