[PATCH] smb: client: validate dacloffset before building DACL pointers

Michael Bommarito
2026-04-20T14:48:03Z

parse_sec_desc(), build_sec_desc(), and the chown path in
id_mode_to_cifs_acl() all add the server-supplied dacloffset to pntsd
before proving a DACL header fits inside the returned security
descriptor.

On 32-bit builds a malicious server can return dacloffset near
U32_MAX, wrap the derived DACL pointer below end_of_acl, and then slip
past the later pointer-based bounds checks. build_sec_desc() and
id_mode_to_cifs_acl() can then dereference DACL fields from the wrapped
pointer in the chmod/chown rewrite paths.

Validate dacloffset numerically before building any DACL pointer and
reuse the same helper at the three DACL entry points.

Fixes: bc3e9dd9d104 ("cifs: Change SIDs in ACEs while transferring file ownership.")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
This applies on top of

[PATCH v2] smb: client: validate the whole DACL before rewriting it
in cifsacl
https://lore.kernel.org/linux-cifs/20260420001131.2865776-1-michael.bommarito@gmail.com/

so that the new dacl_offset_valid() numeric precheck sits upstream of
that series' validate_dacl() structural check at all three call sites.
The two patches are independent fixes for different bug classes on the
same three entry points; applying this one without the KCIFS2 v2 patch
first will fail on the build_sec_desc() hunk because the trailing
context line "rc = validate_dacl(dacl_ptr, end_of_acl)" only exists
after v2. If you prefer a different ordering, happy to reroll on a
plain mainline base instead.

fs/smb/client/cifsacl.c | 35 ++++++++++++++++++++++++++++++++---
1 file changed, 32 insertions(+), 3 deletions(-)

diff --git a/fs/smb/client/cifsacl.c b/fs/smb/client/cifsacl.c
index cb4060ba5e31..87d2a58fc8b4 100644
--- a/fs/smb/client/cifsacl.c
+++ b/fs/smb/client/cifsacl.c
@@ -1263,6 +1263,17 @@ static int parse_sid(struct smb_sid *psid, char *end_of_acl)
return 0;
}

+static bool dacl_offset_valid(unsigned int acl_len, __u32 dacloffset)
+{
+ if (acl_len < sizeof(struct smb_acl))
+ return false;
+
+ if (dacloffset < sizeof(struct smb_ntsd))
+ return false;
+
+ return dacloffset <= acl_len - sizeof(struct smb_acl);
+}
+

/* Convert CIFS ACL to POSIX form */
static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
@@ -1283,7 +1294,6 @@ static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
group_sid_ptr = (struct smb_sid *)((char *)pntsd +
le32_to_cpu(pntsd->gsidoffset));
dacloffset = le32_to_cpu(pntsd->dacloffset);
- dacl_ptr = (struct smb_acl *)((char *)pntsd + dacloffset);
cifs_dbg(NOISY, "revision %d type 0x%x ooffset 0x%x goffset 0x%x sacloffset 0x%x dacloffset 0x%x\n",
pntsd->revision, pntsd->type, le32_to_cpu(pntsd->osidoffset),
le32_to_cpu(pntsd->gsidoffset),
@@ -1314,11 +1324,18 @@ static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
return rc;
}

- if (dacloffset)
+ if (dacloffset) {
+ if (!dacl_offset_valid(acl_len, dacloffset)) {
+ cifs_dbg(VFS, "Server returned illegal DACL offset\n");
+ return -EINVAL;
+ }
+
+ dacl_ptr = (struct smb_acl *)((char *)pntsd + dacloffset);
parse_dacl(dacl_ptr, end_of_acl, owner_sid_ptr,
group_sid_ptr, fattr, get_mode_from_special_sid);
- else
+ } else {
cifs_dbg(FYI, "no ACL\n"); /* BB grant all or default perms? */
+ }

return rc;
}
@@ -1341,6 +1358,11 @@ static int build_sec_desc(struct smb_ntsd *pntsd, struct smb_ntsd *pnntsd,

dacloffset = le32_to_cpu(pntsd->dacloffset);
if (dacloffset) {
+ if (!dacl_offset_valid(secdesclen, dacloffset)) {
+ cifs_dbg(VFS, "Server returned illegal DACL offset\n");
+ return -EINVAL;
+ }
+
dacl_ptr = (struct smb_acl *)((char *)pntsd + dacloffset);
rc = validate_dacl(dacl_ptr, end_of_acl);
if (rc)
@@ -1709,6 +1731,12 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
nsecdesclen = sizeof(struct smb_ntsd) + (sizeof(struct smb_sid) * 2);
dacloffset = le32_to_cpu(pntsd->dacloffset);
if (dacloffset) {
+ if (!dacl_offset_valid(secdesclen, dacloffset)) {
+ cifs_dbg(VFS, "Server returned illegal DACL offset\n");
+ rc = -EINVAL;
+ goto id_mode_to_cifs_acl_exit;
+ }
+
dacl_ptr = (struct smb_acl *)((char *)pntsd + dacloffset);
rc = validate_dacl(dacl_ptr, (char *)pntsd + secdesclen);
if (rc) {
@@ -1751,6 +1779,7 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
rc = ops->set_acl(pnntsd, nsecdesclen, inode, path, aclflag);
cifs_dbg(NOISY, "set_cifs_acl rc: %d\n", rc);
}
+id_mode_to_cifs_acl_exit:
cifs_put_tlink(tlink);

kfree(pnntsd);
--
2.53.0