fix: starter: overwrite glibc's internal tid cache on clone()

Adapted from: opencontainers/runc#4247

Execution of a container using a PID namespace can fail on certain
versions of glibc when Singularity is built with Go 1.22.

This is due to Go 1.22 performing calls using pthread_self which,
from glibc 2.25, is not updated for the current TID on clone.

Fixes sylabs#2677

-----

Original runc explanation:

Since glibc 2.25, the thread-local cache of the current TID is no
longer updated in the child when calling clone(2). This results in
very unfortunate behaviour when Go does pthread calls using
pthread_self(), which has the wrong TID stored.

The "simple" solution is to forcefully overwrite this cached value.
Unfortunately (and unsurprisingly), the layout of "struct pthread"
is strictly private and can change without warning.

Luckily, glibc (currently) uses CLONE_CHILD_CLEARTID for all forks
(with the child_tid set to the cached &PTHREAD_SELF->tid), meaning
that as long as runc is using glibc, when "runc init" is spawned
the child process will have a pointer directly to the cached value
we want to change. With CONFIG_CHECKPOINT_RESTORE=y kernels on
Linux 3.5 and later, we can simply use prctl(PR_GET_TID_ADDRESS).
For older kernels we need to memory scan the TLS structure
(pthread_self() returns a pointer to the start of the structure
so we can "just" scan it for a field containing the current TID
and assume that it is the correct field).

Obviously this is all very horrific, and if you are reading this
in the future, it almost certainly has caused some horrific bug
that I did not forsee. Sorry about that. As far as I can tell,
there is no other workable solution that doesn't also depend on the
CLONE_CHILD_CLEARTID behaviour of glibc in some way. We cannot
"just" do a re-exec after clone(2) for security reasons.

Fixes opencontainers/runc#4233
Signed-off-by: Aleksa Sarai cyphar@cyphar.com

LICENSE_THIRD_PARTY.md

@@ -607,3 +607,31 @@ Contain code from the docker cli project, under the Apache License, Version 2.0.
    limitations under the License.
 */
 ```
+
+## github.com/containers/runc
+
+The source file:
+
+* `cmd/starter/c/starter.c`
+
+Contains code from the runc project, under the Apache License, Version 2.0.
+
+``` text
+runc
+
+Copyright 2012-2015 Docker, Inc.
+
+This product includes software developed at Docker, Inc. (http://www.docker.com).
+
+The following is courtesy of our legal counsel:
+
+
+Use and transfer of Docker may be subject to certain restrictions by the
+United States and other governments.
+It is your responsibility to ensure that your use and/or transfer does not
+violate applicable laws.
+
+For more information, please see http://www.bis.doc.gov
+
+See also http://www.apache.org/dev/crypto.html and/or seek legal counsel.
+```

cmd/starter/c/starter.c

@@ -1,11 +1,28 @@
 /*
-  Copyright (c) 2018-2023, Sylabs, Inc. All rights reserved.
+  Copyright (c) 2018-2024, Sylabs, Inc. All rights reserved.
 
   This software is licensed under a 3-clause BSD license.  Please
   consult LICENSE.md file distributed with the sources of this project regarding
   your rights to use or distribute this software.
-*/
 
+  Portions of this code are adapted from:
+    https://github.com/opencontainers/runc/pull/4247
+
+  Copyright 2012-2015 Docker, Inc.
+
+  This product includes software developed at Docker, Inc. (http://www.docker.com).
+
+  The following is courtesy of our legal counsel:
+
+  Use and transfer of Docker may be subject to certain restrictions by the
+  United States and other governments.
+  It is your responsibility to ensure that your use and/or transfer does not
+  violate applicable laws.
+
+  For more information, please see http://www.bis.doc.gov
+
+  See also http://www.apache.org/dev/crypto.html and/or seek legal counsel.
+*/
 
 #define _GNU_SOURCE
 #include <stdio.h>
@@ -38,6 +55,8 @@
 #include <sys/eventfd.h>
 #include <sys/sysmacros.h>
 #include <linux/magic.h>
+#include <stddef.h>
+#include <pthread.h>
 
 #ifdef SINGULARITY_SECUREBITS
 #  include <linux/securebits.h>
@@ -80,6 +99,233 @@ typedef struct stack {
     char ptr[0];
 } fork_stack_t;
 
+/* tid manipulation code is taken from https://github.com/opencontainers/runc/pull/4247 /*
+
+/* The gettid() libc wrapper was added in glibc 2.30 */
+#if !__GLIBC_PREREQ(2, 30)
+#  if !defined(SYS_gettid) && defined(__NR_gettid)
+#    define SYS_gettid __NR_gettid
+#  endif
+
+pid_t gettid(void) {
+#  ifdef SYS_gettid
+	return syscall(SYS_gettid);
+#  else
+	/* We are single-threaded here, so just using the pid is okay. */
+	return getpid();
+#  endif
+}
+#endif
+
+static pid_t *find_glibc_tls_tid_address(void) {
+    /*
+     * glibc sets CLONE_CHILD_CLEARTID to &THREAD_SELF->tid (the thread-local
+     * cache of the thread's tid), which we can retrieve using
+     * PR_GET_TID_ADDRESS on kernels that support it (Linux >= 3.5 and
+     * CONFIG_CHECKPOINT_RESTORE=y). Otherwise we have to do a somewhat-hairy
+     * linear scan for the address based on pthread_self().
+     *
+     * Other libcs (like musl) set up processes differently, meaning this logic
+     * will only work for runc builds using glibc (more precisely, the process
+     * which spawned "runc init" needs to be a glibc-based process using
+     * glibc's fork() primitives -- this is the case for runc when built with
+     * glibc). The linear scan should still technically work for the musl
+     * versions I've checked, but at the moment we only do this for glibc.
+     */
+
+    pid_t *tid_addr = NULL;
+    pid_t actual_tid = gettid();
+
+    if (!prctl(PR_GET_TID_ADDRESS, &tid_addr)) {
+        /*
+        * Make sure the address actually contains the current TID. musl uses a
+        * different pointer with CLONE_CHILD_CLEARTID, so PR_GET_TID_ADDRESS
+        * succeeding doesn't mean the address is the one we want.
+        */
+        if (tid_addr && *tid_addr == actual_tid) {
+            goto got_tid_addr;
+        }
+    }
+
+    /*
+    * If we cannot use PR_GET_TID_ADDRESS to get &PTHREAD_SELF->tid, we
+    * are probably running on a CONFIG_CHECKPOINT_RESTORE=n kernel.
+    * Unfortunately the layout of "struct pthread" is not public, so we
+    * need to get the address by force.
+    *
+    * So, we treat the structure as though it were pid_t[] to find an
+    * offset whose value matches the tid of the current process. In order
+    * to avoid accidentally choosing an offset in some internal data
+    * structure in tcbhead_t, we first try some known-correct offsets on
+    * the current architecture. If none of those work, we do a linear
+    * scan. Yes, this is *much* worse than PR_GET_TID_ADDRESS and is
+    * pretty terrifying, but we should never get here on the vast majority
+    * of machines.
+    *
+    * (To be honest, maybe it's better to just hope Go doesn't notice any
+    * issues with glibc rather than trying to hack internal glibc
+    * structures to make them "work" with Go. But it seems we need to do
+    * this...)
+    */
+
+    if (tid_addr) {
+        debugf("clone: find_glibc_tls_tid_address: PR_GET_TID_ADDRESS is not the cached tid field (*%p [%d] != %d, pthread_self=%p)",
+            tid_addr, *tid_addr, actual_tid, (void *)pthread_self());
+    } else {
+        debugf("clone: find_glibc_tls_tid_address: PR_GET_TID_ADDRESS failed: %m");
+    }
+
+        warningf("clone: find_glibc_tls_tid_address: falling back to scanning pthread_self(). Please use a kernel with CONFIG_CHECKPOINT_RESTORE=y.");
+
+    /*
+    * These offsets are based on glibc 2.39, but the layout of struct
+    * pthread (at least up to the tid field) has been stable for several
+    * decades. The cached pid (from pre-2.25 glibc) was stored after the
+    * tid field, so even on ancient glibc versions it's "safe" for us to
+    * do this.
+    *
+    * The structure layouts can be found in <sysdeps/.../ntpl/tls.h>.
+    */
+
+    struct tcbhead_t {
+#if defined(__x86_64__)
+        void *tcb, *dtv, *self;
+    	int multiple_threads, gscope_flag;
+	    uintptr_t sysinfo, stack_guard, pointer_guard;
+	    unsigned long int unused_vgetcpu_cache[2];
+	    unsigned int feature_1;
+	    int __glibc_unused1;
+	    void *__private_tm[4];
+	    void *__private_ss;
+	    unsigned long long int ssp_base;
+	    struct {
+    	    int i[4];
+    	} __glibc_unused[8][4] __attribute__((aligned(32)));
+	    void *__padding[8];
+#elif defined(__i386__)
+	    void *tcb, *dtv, *self;
+	    int multiple_threads;
+	    uintptr_t sysinfo, stack_guard, pointer_guard;
+	    int gscope_flag;
+	    unsigned int feature_1;
+	    void *__private_tm[3];
+	    void *__private_ss;
+	    unsigned long ssp_base;
+#elif defined(__powerpc__) || defined(__powerpc64__)
+	    uint64_t hwcap_extn, hwcap;
+#  ifndef __powerpc64__
+    	uint32_t padding, at_platform;
+#  endif
+	    uint32_t __unused;
+#  ifdef __powerpc64__
+	    uint32_t at_platform;
+#  endif
+	    uintptr_t dso_slot2, dso_slot1, tar_save;
+	    void *__private_ss;
+	    uintptr_t ebb_handler, ebb_ctx_pointer, ebb_reserved1, ebb_reserved2, pointer_guard, stack_guard;
+	    void *dtv;
+#elif defined(__s390__) || defined(__s390x__)
+	    void *tcb, *dtv, *self;
+	    int multiple_threads;
+	    uintptr_t sysinfo;
+	    uintptr_t stack_guard;
+	    int gscope_flag;
+	    int __glibc_reserved1;
+	    void *__private_ss;
+#elif defined(__sparc__)
+	    void *tcb, *dtv, *self;
+	    int multiple_threads;
+#  if __WORDSIZE == 64
+	    int gscope_flag;
+#  endif
+	    uintptr_t sysinfo;
+	    uintptr_t stack_guard;
+	    uintptr_t pointer_guard;
+#  if __WORDSIZE != 64
+	    int gscope_flag;
+#  endif
+#else
+	    /* All other architectures have an identical structure. */
+	    void *dtv, *private;
+#endif
+    };
+
+    /* #if TLS_DTV_AT_TP */
+    struct pthread__dtv_at_tp {
+	    union {
+	        struct {
+	            int multiple_threads, gscope_flag;
+	        } header;
+	        void *__padding[24];
+        };
+	    struct {
+	        void *prev, *next;
+	    } list;
+	    pid_t tid;	/* the field we are looking for! */
+    };
+
+    /* #if !TLS_DTV_AT_TP */
+    struct pthread__tcbhead {
+	    union {
+	        struct tcbhead_t header;
+	        void *__padding[24];
+	    };
+	    struct {
+	        void *prev, *next;
+	    } list;
+	    pid_t tid;	/* the field we are looking for! */
+    };
+
+#define TRY_TID_OFFSET(offset)						            \
+    do {								                        \
+	    size_t __idx = (offset);					            \
+	    pid_t *__addr = (pid_t *) (pthread_self() + __idx);		\
+	    if (*__addr == actual_tid) {                            \
+	        tid_addr = __addr;					                \
+	        debugf("clone: find_glibc_tls_tid_address: using %p as tid address (pthread_self+0x%zx, index %zu)\n", \
+	            tid_addr, __idx, __idx / sizeof(pid_t));		\
+	        goto got_tid_addr;					                \
+	    }                                                       \
+    } while (0)
+
+    /* First, try the known-good address offsets. */
+    TRY_TID_OFFSET(offsetof(struct pthread__tcbhead, tid));
+    TRY_TID_OFFSET(offsetof(struct pthread__dtv_at_tp, tid));
+
+    debugf(
+	    "clone: find_glibc_tls_tid_address: known offsets 0x%zx and 0x%zx failed -- falling back to brute-force linear scan\n",
+	    offsetof(struct pthread__dtv_at_tp, tid), offsetof(struct pthread__tcbhead, tid));
+
+    /*
+     * If the known offsets are wrong, we have to fall back to a linear
+     * scan. The pid_t will always be aligned, so we check in blocks of
+     * sizeof(pid_t). This could result in the wrong address, but there
+     * isn't a better option unfortunately.
+     *
+     * On my x86_64 machine, sizeof(struct pthread) is 724. x86_64 has the
+     * largest struct pthread, so scanning up to an offset of 1024 should
+     * cover every architecture without a huge risk of SIGSEGV.
+     */
+    int i;
+    for (i = 0; i < 1024; i += sizeof(pid_t)) {
+	    TRY_TID_OFFSET(i);
+    }
+
+got_tid_addr:
+    if (!tid_addr) {
+	    warningf("clone: find_glibc_tls_tid_address: could not find tid address\n");
+    } else if (*tid_addr != actual_tid) {
+	    warningf(
+	        "clone: find_glibc_tls_tid_address: glibc private tid address is wrong: *%p [%d] != gettid() %d\n",
+	        tid_addr, *tid_addr, actual_tid);
+    } else {
+	    debugf(
+	        "clone: find_glibc_tls_tid_address: found seemingly viable tid address %p (pthread_self=%p)\n",
+	        tid_addr, (void *)pthread_self());
+    }
+    return tid_addr;
+}
+
 /*
  * fork_ns child stack living in BSS section, instead of
  * allocating dynamically the stack during each call, a stack
@@ -104,11 +350,54 @@ __attribute__ ((returns_twice)) __attribute__((noinline)) static int fork_ns(uns
      * continue execution from the calling point.
      */
     if ( sigsetjmp(env, 1) ) {
-        /* child process will return here after siglongjmp call in clone_fn */
-        return 0;
+	    /* child process will return here after siglongjmp call in clone_fn */
+	    return 0;
     }
+
     /* parent process */
-    return clone(clone_fn, child_stack.ptr, (SIGCHLD|flags), env);
+
+    /*
+    * tid manipulation code is taken from https://github.com/opencontainers/runc/pull/4247
+    *
+    * Since glibc 2.25 (see c579f48edba88380635ab98cb612030e3ed8691e),
+    * glibc no longer updates the TLS state containing the current process
+    * tid after clone(2). This results in stale TIDs being used when Go
+    * 1.22 and later call pthread_gettattr_np(pthread_self()), resulting
+    * in crashes on ancient glibcs and errors on newer glibcs.
+    *
+    * Luckily, because the address containing pthread's cached TID is also
+    * used for CLONE_CHILD_CLEARTID, we can poke around in glibc's internal
+    * cache by getting the address using PR_GET_TID_ADDRESS. For kernels
+    * without PR_GET_TID_ADDRESS support (Linux < 3.5 or
+    * CONFIG_CHECKPOINT_RESTORE=n), we have to do some far uglier tricks to
+    * find the address. We then overwrite the address with the correct TID
+    * using CLONE_CHILD_SETTID, and set CLONE_CHILD_CLEARTID to match glibc's
+    * arch_fork() (which also allows descendants to find the address with
+    * PR_GET_TID_ADDRESS).
+    *
+    * Yes, this is pretty horrific, but the core issue here is that we
+    * need to run Go code ("runc init") in the child after fork(), which
+    * is not allowed by glibc (see signal-safety(7)). We cannot exec to
+    * solve the problem because we are in a security critical situation
+    * here, and doing an exec would allow for container escapes (obvious
+    * issues include that the shared libraries loaded from a re-exec would
+    * come from the container, and doing an exec here would reset mm->user_ns
+    * which would allow for breakouts by userns containers with
+    * SYS_CAP_PTRACE).
+    *
+    * Note that all of this is only guaranteed to work if "runc init" was
+    * spawned from a *glibc* fork. A fork from another libc might not work, so
+    * we only do this for glibc.
+    */
+
+    pid_t *tid_addr = NULL;
+#ifdef __GLIBC__
+    tid_addr = find_glibc_tls_tid_address();
+#endif
+
+    return clone(clone_fn, child_stack.ptr,
+	    CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID | SIGCHLD | flags, env,
+	    NULL /* parent_tid */ , NULL /* tls */, tid_addr /* child_tid */);
 }
 
 static void priv_escalate(bool keep_fsuid) {
@@ -406,7 +695,7 @@ static void set_rpc_privileges(void) {
         priv->capabilities.bounding |= capflag(CAP_SYS_CHROOT);
         priv->capabilities.bounding |= capflag(CAP_SYS_PTRACE);
     }
-    
+
 
     debugf("Set RPC privileges\n");
     apply_privileges(priv, current);
@@ -1309,7 +1598,7 @@ __attribute__((constructor)) static void init(void) {
             }
             // Close master end of post start socket
             close(post_start_socket[0]);
-        
+
             set_parent_death_signal(SIGTERM);
             verbosef("Spawn PostStartHost\n");
             goexecute = POST_START_HOST;
@@ -1336,7 +1625,7 @@ __attribute__((constructor)) static void init(void) {
             }
             // Close master end of cleanup socket
             close(cleanup_socket[0]);
-        
+
             set_parent_death_signal(SIGTERM);
             verbosef("Spawn CleanupHost\n");
             goexecute = CLEANUP_HOST;