[PATCH] ksmbd: fix SID memory leak in set_posix_acl_entries_dacl() on overflow

[PATCH] ksmbd: fix SID memory leak in set_posix_acl_entries_dacl() on overflow

[Ferry Meng]

Commit 299f962c0b02 ("ksmbd: use check_add_overflow() to prevent u16
DACL size overflow") added check_add_overflow() guards that break out
of the ACE-building loops in set_posix_acl_entries_dacl() when the
accumulated DACL size would wrap past 65535.

However, each iteration allocates a struct smb_sid via kmalloc_obj()
at the top of the loop and relies on the kfree(sid) call at the end
of the loop body (the 'pass_same_sid' label in the first loop, and
the explicit kfree at the tail of the second loop) to release it.
The newly introduced 'break' statements bypass those kfree() calls,
leaking the sid buffer every time an overflow is detected.

A malicious or malformed file with enough POSIX ACL entries to trip
the overflow check will leak one or more struct smb_sid allocations
on every request that touches the file's DACL, providing a trivial
kernel memory exhaustion vector.

Free sid before breaking out of the loops to plug the leak.

Fixes: 299f962c0b02 ("ksmbd: use check_add_overflow() to prevent u16 DACL size overflow")
Cc: stable@vger.kernel.org
Signed-off-by: Ferry Meng <mengferry@linux.alibaba.com>
---
 fs/smb/server/smbacl.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/smb/server/smbacl.c b/fs/smb/server/smbacl.c
index c1d1f34581d6..9161e9d7ed24 100644
--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -643,8 +643,10 @@ static void set_posix_acl_entries_dacl(struct mnt_idmap *idmap,
 		ntace = (struct smb_ace *)((char *)pndace + *size);
 		ace_sz = fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED, flags,
 				pace->e_perm, 0777);
-		if (check_add_overflow(*size, ace_sz, size))
+		if (check_add_overflow(*size, ace_sz, size)) {
+			kfree(sid);
 			break;
+		}
 		(*num_aces)++;
 		if (pace->e_tag == ACL_USER)
 			ntace->access_req |=
@@ -655,8 +657,10 @@ static void set_posix_acl_entries_dacl(struct mnt_idmap *idmap,
 			ntace = (struct smb_ace *)((char *)pndace + *size);
 			ace_sz = fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED,
 					0x03, pace->e_perm, 0777);
-			if (check_add_overflow(*size, ace_sz, size))
+			if (check_add_overflow(*size, ace_sz, size)) {
+				kfree(sid);
 				break;
+			}
 			(*num_aces)++;
 			if (pace->e_tag == ACL_USER)
 				ntace->access_req |=
@@ -698,8 +702,10 @@ static void set_posix_acl_entries_dacl(struct mnt_idmap *idmap,
 		ntace = (struct smb_ace *)((char *)pndace + *size);
 		ace_sz = fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED, 0x0b,
 				pace->e_perm, 0777);
-		if (check_add_overflow(*size, ace_sz, size))
+		if (check_add_overflow(*size, ace_sz, size)) {
+			kfree(sid);
 			break;
+		}
 		(*num_aces)++;
 		if (pace->e_tag == ACL_USER)
 			ntace->access_req |=
-- 
2.43.5

Re: [PATCH] ksmbd: fix SID memory leak in set_posix_acl_entries_dacl() on overflow

[Namjae Jeon]

On Mon, May 11, 2026 at 10:18 PM Ferry Meng <mengferry@linux.alibaba.com> wrote:
>
> Commit 299f962c0b02 ("ksmbd: use check_add_overflow() to prevent u16
> DACL size overflow") added check_add_overflow() guards that break out
> of the ACE-building loops in set_posix_acl_entries_dacl() when the
> accumulated DACL size would wrap past 65535.
>
> However, each iteration allocates a struct smb_sid via kmalloc_obj()
> at the top of the loop and relies on the kfree(sid) call at the end
> of the loop body (the 'pass_same_sid' label in the first loop, and
> the explicit kfree at the tail of the second loop) to release it.
> The newly introduced 'break' statements bypass those kfree() calls,
> leaking the sid buffer every time an overflow is detected.
>
> A malicious or malformed file with enough POSIX ACL entries to trip
> the overflow check will leak one or more struct smb_sid allocations
> on every request that touches the file's DACL, providing a trivial
> kernel memory exhaustion vector.
>
> Free sid before breaking out of the loops to plug the leak.
>
> Fixes: 299f962c0b02 ("ksmbd: use check_add_overflow() to prevent u16 DACL size overflow")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ferry Meng <mengferry@linux.alibaba.com>
Applied it to #ksmbd-for-next-next.
Thanks!