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exec: do not sleep in TASK_TRACED under ->cred_guard_mutex

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Tom Horsley reports that his debugger hangs when it tries to read
/proc/pid_of_tracee/maps, this happens since

	"mm_for_maps: take ->cred_guard_mutex to fix the race with exec"
	04b836cbf19e885f8366bccb2e4b0474346c02d

commit in 2.6.31.

But the root of the problem lies in the fact that do_execve() path calls
tracehook_report_exec() which can stop if the tracer sets PT_TRACE_EXEC.

The tracee must not sleep in TASK_TRACED holding this mutex.  Even if we
remove ->cred_guard_mutex from mm_for_maps() and proc_pid_attr_write(),
another task doing PTRACE_ATTACH should not hang until it is killed or the
tracee resumes.

With this patch do_execve() does not use ->cred_guard_mutex directly and
we do not hold it throughout, instead:

	- introduce prepare_bprm_creds() helper, it locks the mutex
	  and calls prepare_exec_creds() to initialize bprm->cred.

	- install_exec_creds() drops the mutex after commit_creds(),
	  and thus before tracehook_report_exec()->ptrace_stop().

	  or, if exec fails,

	  free_bprm() drops this mutex when bprm->cred != NULL which
	  indicates install_exec_creds() was not called.

Reported-by: Tom Horsley <tom.horsley@att.net>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2009-09-05 11:17:13 -07:00 • committed by Linus Torvalds
commit a2a8474c3f
3 changed files with 43 additions and 38 deletions
Download patch file Download diff file Expand all files Collapse all files

17
fs/compat.c
View file

@ -1485,20 +1485,15 @@ int compat_do_execve(char * filename,
if (!bprm)
goto out_files;
retval = -ERESTARTNOINTR;
if (mutex_lock_interruptible(&current->cred_guard_mutex))
retval = prepare_bprm_creds(bprm);
if (retval)
goto out_free;
current->in_execve = 1;
retval = -ENOMEM;
bprm->cred = prepare_exec_creds();
if (!bprm->cred)
goto out_unlock;
retval = check_unsafe_exec(bprm);
if (retval < 0)
goto out_unlock;
goto out_free;
clear_in_exec = retval;
current->in_execve = 1;
file = open_exec(filename);
retval = PTR_ERR(file);
@ -1547,7 +1542,6 @@ int compat_do_execve(char * filename,
/* execve succeeded */
current->fs->in_exec = 0;
current->in_execve = 0;
mutex_unlock(&current->cred_guard_mutex);
acct_update_integrals(current);
free_bprm(bprm);
if (displaced)
@ -1567,10 +1561,7 @@ out_file:
out_unmark:
if (clear_in_exec)
current->fs->in_exec = 0;
out_unlock:
current->in_execve = 0;
mutex_unlock(&current->cred_guard_mutex);
out_free:
free_bprm(bprm);

63
fs/exec.c
View file

@ -1015,6 +1015,35 @@ out:
EXPORT_SYMBOL(flush_old_exec);
/*
* Prepare credentials and lock ->cred_guard_mutex.
* install_exec_creds() commits the new creds and drops the lock.
* Or, if exec fails before, free_bprm() should release ->cred and
* and unlock.
*/
int prepare_bprm_creds(struct linux_binprm *bprm)
{
if (mutex_lock_interruptible(&current->cred_guard_mutex))
return -ERESTARTNOINTR;
bprm->cred = prepare_exec_creds();
if (likely(bprm->cred))
return 0;
mutex_unlock(&current->cred_guard_mutex);
return -ENOMEM;
}
void free_bprm(struct linux_binprm *bprm)
{
free_arg_pages(bprm);
if (bprm->cred) {
mutex_unlock(&current->cred_guard_mutex);
abort_creds(bprm->cred);
}
kfree(bprm);
}
/*
* install the new credentials for this executable
*/
@ -1024,12 +1053,13 @@ void install_exec_creds(struct linux_binprm *bprm)
commit_creds(bprm->cred);
bprm->cred = NULL;
/* cred_guard_mutex must be held at least to this point to prevent
/*
* cred_guard_mutex must be held at least to this point to prevent
* ptrace_attach() from altering our determination of the task's
* credentials; any time after this it may be unlocked */
* credentials; any time after this it may be unlocked.
*/
security_bprm_committed_creds(bprm);
mutex_unlock(&current->cred_guard_mutex);
}
EXPORT_SYMBOL(install_exec_creds);
@ -1246,14 +1276,6 @@ int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
EXPORT_SYMBOL(search_binary_handler);
void free_bprm(struct linux_binprm *bprm)
{
free_arg_pages(bprm);
if (bprm->cred)
abort_creds(bprm->cred);
kfree(bprm);
}
/*
* sys_execve() executes a new program.
*/
@ -1277,20 +1299,15 @@ int do_execve(char * filename,
if (!bprm)
goto out_files;
retval = -ERESTARTNOINTR;
if (mutex_lock_interruptible(&current->cred_guard_mutex))
retval = prepare_bprm_creds(bprm);
if (retval)
goto out_free;
current->in_execve = 1;
retval = -ENOMEM;
bprm->cred = prepare_exec_creds();
if (!bprm->cred)
goto out_unlock;
retval = check_unsafe_exec(bprm);
if (retval < 0)
goto out_unlock;
goto out_free;
clear_in_exec = retval;
current->in_execve = 1;
file = open_exec(filename);
retval = PTR_ERR(file);
@ -1340,7 +1357,6 @@ int do_execve(char * filename,
/* execve succeeded */
current->fs->in_exec = 0;
current->in_execve = 0;
mutex_unlock(&current->cred_guard_mutex);
acct_update_integrals(current);
free_bprm(bprm);
if (displaced)
@ -1360,10 +1376,7 @@ out_file:
out_unmark:
if (clear_in_exec)
current->fs->in_exec = 0;
out_unlock:
current->in_execve = 0;
mutex_unlock(&current->cred_guard_mutex);
out_free:
free_bprm(bprm);

1
include/linux/binfmts.h
View file

@ -117,6 +117,7 @@ extern int setup_arg_pages(struct linux_binprm * bprm,
int executable_stack);
extern int bprm_mm_init(struct linux_binprm *bprm);
extern int copy_strings_kernel(int argc,char ** argv,struct linux_binprm *bprm);
extern int prepare_bprm_creds(struct linux_binprm *bprm);
extern void install_exec_creds(struct linux_binprm *bprm);
extern void do_coredump(long signr, int exit_code, struct pt_regs *regs);
extern int set_binfmt(struct linux_binfmt *new);