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Commit a2a8474c authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
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exec: do not sleep in TASK_TRACED under ->cred_guard_mutex 



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: default avatarTom Horsley <tom.horsley@att.net>
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Acked-by: default avatarDavid Howells <dhowells@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent dd5d241e

fs/compat.c

+4 −13
Original line number Original line Diff line number Diff line
@@ -1485,20 +1485,15 @@ int compat_do_execve(char * filename, 
	if (!bprm)

	if (!bprm)

		goto out_files;

		goto out_files;





	retval = -ERESTARTNOINTR;

	retval = prepare_bprm_creds(bprm);

	if (mutex_lock_interruptible(&current->cred_guard_mutex))

	if (retval)

		goto out_free;

		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);

	retval = check_unsafe_exec(bprm);

	if (retval < 0)

	if (retval < 0)

		goto out_unlock;

		goto out_free;

	clear_in_exec = retval;

	clear_in_exec = retval;

	current->in_execve = 1;





	file = open_exec(filename);

	file = open_exec(filename);

	retval = PTR_ERR(file);

	retval = PTR_ERR(file);
@@ -1547,7 +1542,6 @@ int compat_do_execve(char * filename, 
	/* execve succeeded */

	/* execve succeeded */

	current->fs->in_exec = 0;

	current->fs->in_exec = 0;

	current->in_execve = 0;

	current->in_execve = 0;

	mutex_unlock(&current->cred_guard_mutex);

	acct_update_integrals(current);

	acct_update_integrals(current);

	free_bprm(bprm);

	free_bprm(bprm);

	if (displaced)

	if (displaced)
@@ -1567,10 +1561,7 @@ int compat_do_execve(char * filename, 
out_unmark:

out_unmark:

	if (clear_in_exec)

	if (clear_in_exec)

		current->fs->in_exec = 0;

		current->fs->in_exec = 0;



out_unlock:

	current->in_execve = 0;

	current->in_execve = 0;

	mutex_unlock(&current->cred_guard_mutex);





out_free:

out_free:

	free_bprm(bprm);

	free_bprm(bprm);

fs/exec.c

+38 −25
Original line number Original line Diff line number Diff line
@@ -1015,6 +1015,35 @@ int flush_old_exec(struct linux_binprm * bprm) 




EXPORT_SYMBOL(flush_old_exec);

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

 * install the new credentials for this executable

 */

 */
@@ -1024,12 +1053,13 @@ void install_exec_creds(struct linux_binprm *bprm) 




	commit_creds(bprm->cred);

	commit_creds(bprm->cred);

	bprm->cred = NULL;

	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

	 * 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);

	security_bprm_committed_creds(bprm);

	mutex_unlock(&current->cred_guard_mutex);

}

}

EXPORT_SYMBOL(install_exec_creds);

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);

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.

 * sys_execve() executes a new program.

 */

 */
@@ -1277,20 +1299,15 @@ int do_execve(char * filename, 
	if (!bprm)

	if (!bprm)

		goto out_files;

		goto out_files;





	retval = -ERESTARTNOINTR;

	retval = prepare_bprm_creds(bprm);

	if (mutex_lock_interruptible(&current->cred_guard_mutex))

	if (retval)

		goto out_free;

		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);

	retval = check_unsafe_exec(bprm);

	if (retval < 0)

	if (retval < 0)

		goto out_unlock;

		goto out_free;

	clear_in_exec = retval;

	clear_in_exec = retval;

	current->in_execve = 1;





	file = open_exec(filename);

	file = open_exec(filename);

	retval = PTR_ERR(file);

	retval = PTR_ERR(file);
@@ -1340,7 +1357,6 @@ int do_execve(char * filename, 
	/* execve succeeded */

	/* execve succeeded */

	current->fs->in_exec = 0;

	current->fs->in_exec = 0;

	current->in_execve = 0;

	current->in_execve = 0;

	mutex_unlock(&current->cred_guard_mutex);

	acct_update_integrals(current);

	acct_update_integrals(current);

	free_bprm(bprm);

	free_bprm(bprm);

	if (displaced)

	if (displaced)
@@ -1360,10 +1376,7 @@ int do_execve(char * filename, 
out_unmark:

out_unmark:

	if (clear_in_exec)

	if (clear_in_exec)

		current->fs->in_exec = 0;

		current->fs->in_exec = 0;



out_unlock:

	current->in_execve = 0;

	current->in_execve = 0;

	mutex_unlock(&current->cred_guard_mutex);





out_free:

out_free:

	free_bprm(bprm);

	free_bprm(bprm);

include/linux/binfmts.h

+1 −0
Original line number Original line Diff line number Diff line
@@ -117,6 +117,7 @@ extern int setup_arg_pages(struct linux_binprm * bprm, 
			   int executable_stack);

			   int executable_stack);

extern int bprm_mm_init(struct linux_binprm *bprm);

extern int bprm_mm_init(struct linux_binprm *bprm);

extern int copy_strings_kernel(int argc,char ** argv,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 install_exec_creds(struct linux_binprm *bprm);

extern void do_coredump(long signr, int exit_code, struct pt_regs *regs);

extern void do_coredump(long signr, int exit_code, struct pt_regs *regs);

extern int set_binfmt(struct linux_binfmt *new);

extern int set_binfmt(struct linux_binfmt *new);