AddressSanitizer: null pointer dereference (SEGV) in Sass::Expand::operator (libsass/src/expand.cpp:249:28)

[glen-mac]

Hey there, I have discovered a null pointer dereference in libsass at: expand.cpp:249:28

Found when fuzzing commit 60f8391 of libsass, using commit aa6d5c6 of sassc as a harness.

Compile flags to reproduce:

CC=clang CXX=clang++ CFLAGS='-fsanitize=address -g -O2 -fno-omit-frame-pointer' CXXFLAGS=$CFLAGS make -C sassc -j8

System information:

$ uname -a
Linux s127422 3.13.0-137-generic #186-Ubuntu SMP Mon Dec 4 19:09:19 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

This bug was found to be in libsass releases from 3.5.0 through to 3.5.2 and then re-introduced in the commit 60f8391.

You can find a collection of PoC files that trigger the bug here.

The full ASAN report is shown below:

↳ ./sassc.bin < crash.file
ASAN:DEADLYSIGNAL
=================================================================
==6006==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x00000071bb37 bp 0x7ffee8b515d0 sp 0x7ffee8b51140 T0)
    #0 0x71bb36 in Sass::Expand::operator()(Sass::Declaration*) /home/glenn/temp/libsass/src/expand.cpp:258:28
    #1 0x731326 in Sass::Expand::append_block(Sass::Block*) /home/glenn/temp/libsass/src/expand.cpp:808:27
    #2 0x714f0a in Sass::Expand::operator()(Sass::Block*) /home/glenn/temp/libsass/src/expand.cpp:81:5
    #3 0x717116 in Sass::Expand::operator()(Sass::Ruleset*) /home/glenn/temp/libsass/src/expand.cpp:153:27
    #4 0x731326 in Sass::Expand::append_block(Sass::Block*) /home/glenn/temp/libsass/src/expand.cpp:808:27
    #5 0x714f0a in Sass::Expand::operator()(Sass::Block*) /home/glenn/temp/libsass/src/expand.cpp:81:5
    #6 0x5521f7 in Sass::Context::compile() /home/glenn/temp/libsass/src/context.cpp:659:12
    #7 0x55144c in Sass::Data_Context::parse() /home/glenn/temp/libsass/src/context.cpp:630:12
    #8 0x5200ca in Sass::sass_parse_block(Sass_Compiler*) /home/glenn/temp/libsass/src/sass_context.cpp:238:22
    #9 0x5200ca in sass_compiler_parse /home/glenn/temp/libsass/src/sass_context.cpp:487
    #10 0x51f6d9 in sass_compile_context(Sass_Context*, Sass::Context*) /home/glenn/temp/libsass/src/sass_context.cpp:375:7
    #11 0x51707d in compile_stdin /home/glenn/temp/findsass/sassc/sassc.c:138:5
    #12 0x517a8e in main /home/glenn/temp/findsass/sassc/sassc.c:375:18
    #13 0x7f6ce042a82f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/../csu/libc-start.c:291
    #14 0x4456d8 in _start (/home/glenn/temp/findsass/sassc.bin+0x4456d8)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/glenn/temp/libsass/src/expand.cpp:258:28 in Sass::Expand::operator()(Sass::Declaration*)
==6006==ABORTING

[xzyfer]

Fixed in 0bc35e3.

Thanks.