[PATCH] fanotify: Fix stale file descriptor in copy_event_to_user()

[PATCH] fanotify: Fix stale file descriptor in copy_event_to_user()

[Dan Carpenter]

This code calls fd_install() which gives the userspace access to the fd.
Then if copy_info_records_to_user() fails it calls put_unused_fd(fd) but
that will not release it and leads to a stale entry in the file
descriptor table.

Generally you can't trust the fd after a call to fd_install().  The fix
is to delay the fd_install() until everything else has succeeded.

Fortunately it requires CAP_SYS_ADMIN to reach this code so the security
impact is less.

Fixes: f644bc449b37 ("fanotify: fix copy_event_to_user() fid error clean up")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mathias Krause <minipli@grsecurity.net>
---
 fs/notify/fanotify/fanotify_user.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 1026f67b1d1e..2ff6bd85ba8f 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -701,9 +701,6 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 	if (fanotify_is_perm_event(event->mask))
 		FANOTIFY_PERM(event)->fd = fd;
 
-	if (f)
-		fd_install(fd, f);
-
 	if (info_mode) {
 		ret = copy_info_records_to_user(event, info, info_mode, pidfd,
 						buf, count);
@@ -711,6 +708,9 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 			goto out_close_fd;
 	}
 
+	if (f)
+		fd_install(fd, f);
+
 	return metadata.event_len;
 
 out_close_fd:
-- 
2.20.1

Re: [PATCH] fanotify: Fix stale file descriptor in copy_event_to_user()

[Jan Kara]

On Fri 28-01-22 22:57:01, Dan Carpenter wrote:
> This code calls fd_install() which gives the userspace access to the fd.
> Then if copy_info_records_to_user() fails it calls put_unused_fd(fd) but
> that will not release it and leads to a stale entry in the file
> descriptor table.
> 
> Generally you can't trust the fd after a call to fd_install().  The fix
> is to delay the fd_install() until everything else has succeeded.
> 
> Fortunately it requires CAP_SYS_ADMIN to reach this code so the security
> impact is less.
> 
> Fixes: f644bc449b37 ("fanotify: fix copy_event_to_user() fid error clean up")
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> Reviewed-by: Mathias Krause <minipli@grsecurity.net>

Thanks. I've added the patch to my tree and will push it to Linus.

								Honza

> ---
>  fs/notify/fanotify/fanotify_user.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 1026f67b1d1e..2ff6bd85ba8f 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -701,9 +701,6 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>  	if (fanotify_is_perm_event(event->mask))
>  		FANOTIFY_PERM(event)->fd = fd;
>  
> -	if (f)
> -		fd_install(fd, f);
> -
>  	if (info_mode) {
>  		ret = copy_info_records_to_user(event, info, info_mode, pidfd,
>  						buf, count);
> @@ -711,6 +708,9 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>  			goto out_close_fd;
>  	}
>  
> +	if (f)
> +		fd_install(fd, f);
> +
>  	return metadata.event_len;
>  
>  out_close_fd:
> -- 
> 2.20.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR