[PATCH] fs: correctly check for errors from replace_fd() in receive_fd_replace()

[PATCH] fs: correctly check for errors from replace_fd() in receive_fd_replace()

[Thomas Weißschuh]

replace_fd() returns either a negative error number or the number of the
new file descriptor. The current code misinterprets any positive file
descriptor number as an error.

Only check for negative error numbers, so that __receive_sock() is called
correctly for valid file descriptors.

Fixes: 173817151b15 ("fs: Expand __receive_fd() to accept existing fd")
Fixes: 42eb0d54c08a ("fs: split receive_fd_replace from __receive_fd")
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
Untested, it stuck out while reading the code.
---
 fs/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/file.c b/fs/file.c
index 6d2275c3be9c6967d16c75d1b6521f9b58980926..56c3a045121d8f43a54cf05e6ce1962f896339ac 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1387,7 +1387,7 @@ int receive_fd_replace(int new_fd, struct file *file, unsigned int o_flags)
 	if (error)
 		return error;
 	error = replace_fd(new_fd, file, o_flags);
-	if (error)
+	if (error < 0)
 		return error;
 	__receive_sock(file);
 	return new_fd;

---
base-commit: 89748acdf226fd1a8775ff6fa2703f8412b286c8
change-id: 20250801-fix-receive_fd_replace-7fdd5ce6532d

Best regards,
-- 
Thomas Weißschuh <thomas.weissschuh@linutronix.de>

Re: [PATCH] fs: correctly check for errors from replace_fd() in receive_fd_replace()

[Jan Kara]

On Fri 01-08-25 09:38:38, Thomas Weißschuh wrote:
> replace_fd() returns either a negative error number or the number of the
> new file descriptor. The current code misinterprets any positive file
> descriptor number as an error.
> 
> Only check for negative error numbers, so that __receive_sock() is called
> correctly for valid file descriptors.
> 
> Fixes: 173817151b15 ("fs: Expand __receive_fd() to accept existing fd")
> Fixes: 42eb0d54c08a ("fs: split receive_fd_replace from __receive_fd")
> Cc: stable@vger.kernel.org
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

Indeed. I'm wondering how come nobody noticed... Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
> Untested, it stuck out while reading the code.
> ---
>  fs/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 6d2275c3be9c6967d16c75d1b6521f9b58980926..56c3a045121d8f43a54cf05e6ce1962f896339ac 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -1387,7 +1387,7 @@ int receive_fd_replace(int new_fd, struct file *file, unsigned int o_flags)
>  	if (error)
>  		return error;
>  	error = replace_fd(new_fd, file, o_flags);
> -	if (error)
> +	if (error < 0)
>  		return error;
>  	__receive_sock(file);
>  	return new_fd;
> 
> ---
> base-commit: 89748acdf226fd1a8775ff6fa2703f8412b286c8
> change-id: 20250801-fix-receive_fd_replace-7fdd5ce6532d
> 
> Best regards,
> -- 
> Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] fs: correctly check for errors from replace_fd() in receive_fd_replace()

[Al Viro]

On Fri, Aug 01, 2025 at 12:48:09PM +0200, Jan Kara wrote:
> On Fri 01-08-25 09:38:38, Thomas Weißschuh wrote:
> > replace_fd() returns either a negative error number or the number of the
> > new file descriptor. The current code misinterprets any positive file
> > descriptor number as an error.
> > 
> > Only check for negative error numbers, so that __receive_sock() is called
> > correctly for valid file descriptors.
> > 
> > Fixes: 173817151b15 ("fs: Expand __receive_fd() to accept existing fd")
> > Fixes: 42eb0d54c08a ("fs: split receive_fd_replace from __receive_fd")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> 
> Indeed. I'm wondering how come nobody noticed...

One word: seccomp.  Considering the background amount of bogus userland
behaviour coming with it, I wouldn't expect a... vigorous test coverage
for that one ;-/

It's definitely a bug that needs fixing, but I'm not sure this is the right
way to fix it.

Look: replace_fd(fd, file, flags) returns fd on success and -E... on failure.
Not a single user cares which non-negative value had been returned.  What's
more, "returns fd on success" is a side effect of using do_dup2() and
being lazy about it.

And the entire thing is not on any hot paths.  So I suspect that a better fix
would be

	err = do_dup2(files, file, fd, flags);
	if (err < 0)
		return err;
	return 0;

in replace_fd() in place of
	return do_dup2(files, file, fd, flags);

so we don't invite more surprises like that.

Re: [PATCH] fs: correctly check for errors from replace_fd() in receive_fd_replace()

[Christian Brauner]

On Fri, Aug 01, 2025 at 09:38:38AM +0200, Thomas Weißschuh wrote:
> replace_fd() returns either a negative error number or the number of the
> new file descriptor. The current code misinterprets any positive file
> descriptor number as an error.
> 
> Only check for negative error numbers, so that __receive_sock() is called
> correctly for valid file descriptors.
> 
> Fixes: 173817151b15 ("fs: Expand __receive_fd() to accept existing fd")
> Fixes: 42eb0d54c08a ("fs: split receive_fd_replace from __receive_fd")
> Cc: stable@vger.kernel.org
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> ---
> Untested, it stuck out while reading the code.
> ---
>  fs/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 6d2275c3be9c6967d16c75d1b6521f9b58980926..56c3a045121d8f43a54cf05e6ce1962f896339ac 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -1387,7 +1387,7 @@ int receive_fd_replace(int new_fd, struct file *file, unsigned int o_flags)
>  	if (error)
>  		return error;
>  	error = replace_fd(new_fd, file, o_flags);
> -	if (error)
> +	if (error < 0)
>  		return error;

What in the holy fsck? Why did the seccomp selftests not fail
horrendously explode because of that.