[PATCH 5.15.y] smb: client: validate the whole DACL before rewriting it in cifsacl

[Sasha Levin <sashal@kernel.org>]

From: Michael Bommarito <michael.bommarito@gmail.com>

[ Upstream commit 0a8cf165566ba55a39fd0f4de172119dd646d39a ]

build_sec_desc() and id_mode_to_cifs_acl() derive a DACL pointer from a
server-supplied dacloffset and then use the incoming ACL to rebuild the
chmod/chown security descriptor.

The original fix only checked that the struct smb_acl header fits before
reading dacl_ptr->size or dacl_ptr->num_aces.  That avoids the immediate
header-field OOB read, but the rewrite helpers still walk ACEs based on
pdacl->num_aces with no structural validation of the incoming DACL body.

A malicious server can return a truncated DACL that still contains a
header, claims one or more ACEs, and then drive
replace_sids_and_copy_aces() or set_chmod_dacl() past the validated
extent while they compare or copy attacker-controlled ACEs.

Factor the DACL structural checks into validate_dacl(), extend them to
validate each ACE against the DACL bounds, and use the shared validator
before the chmod/chown rebuild paths.  parse_dacl() reuses the same
validator so the read-side parser and write-side rewrite paths agree on
what constitutes a well-formed incoming DACL.

Fixes: bc3e9dd9d104 ("cifs: Change SIDs in ACEs while transferring file ownership.")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Assisted-by: Codex:gpt-5-4
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
[ renamed smb_acl/smb_ace/smb_sid/smb_ntsd to cifs_* and widened num_aces from u16 to u32 for 6.1's __le32 field ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/cifs/cifsacl.c | 95 ++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 85 insertions(+), 10 deletions(-)

diff --git a/fs/cifs/cifsacl.c b/fs/cifs/cifsacl.c
index bf861fef2f0c3..b6dc3543a40f8 100644
--- a/fs/cifs/cifsacl.c
+++ b/fs/cifs/cifsacl.c
@@ -753,6 +753,78 @@ static void dump_ace(struct cifs_ace *pace, char *end_of_acl)
 }
 #endif
 
+static int validate_dacl(struct cifs_acl *pdacl, char *end_of_acl)
+{
+	int i, ace_hdr_size, ace_size, min_ace_size;
+	u16 dacl_size;
+	u32 num_aces;
+	char *acl_base, *end_of_dacl;
+	struct cifs_ace *pace;
+
+	if (!pdacl)
+		return 0;
+
+	if (end_of_acl < (char *)pdacl + sizeof(struct cifs_acl)) {
+		cifs_dbg(VFS, "ACL too small to parse DACL\n");
+		return -EINVAL;
+	}
+
+	dacl_size = le16_to_cpu(pdacl->size);
+	if (dacl_size < sizeof(struct cifs_acl) ||
+	    end_of_acl < (char *)pdacl + dacl_size) {
+		cifs_dbg(VFS, "ACL too small to parse DACL\n");
+		return -EINVAL;
+	}
+
+	num_aces = le32_to_cpu(pdacl->num_aces);
+	if (!num_aces)
+		return 0;
+
+	ace_hdr_size = offsetof(struct cifs_ace, sid) +
+		offsetof(struct cifs_sid, sub_auth);
+	min_ace_size = ace_hdr_size + sizeof(__le32);
+	if (num_aces > (dacl_size - sizeof(struct cifs_acl)) / min_ace_size) {
+		cifs_dbg(VFS, "ACL too small to parse DACL\n");
+		return -EINVAL;
+	}
+
+	end_of_dacl = (char *)pdacl + dacl_size;
+	acl_base = (char *)pdacl;
+	ace_size = sizeof(struct cifs_acl);
+
+	for (i = 0; i < num_aces; ++i) {
+		if (end_of_dacl - acl_base < ace_size) {
+			cifs_dbg(VFS, "ACL too small to parse ACE\n");
+			return -EINVAL;
+		}
+
+		pace = (struct cifs_ace *)(acl_base + ace_size);
+		acl_base = (char *)pace;
+
+		if (end_of_dacl - acl_base < ace_hdr_size ||
+		    pace->sid.num_subauth == 0 ||
+		    pace->sid.num_subauth > SID_MAX_SUB_AUTHORITIES) {
+			cifs_dbg(VFS, "ACL too small to parse ACE\n");
+			return -EINVAL;
+		}
+
+		ace_size = ace_hdr_size + sizeof(__le32) * pace->sid.num_subauth;
+		if (end_of_dacl - acl_base < ace_size ||
+		    le16_to_cpu(pace->size) < ace_size) {
+			cifs_dbg(VFS, "ACL too small to parse ACE\n");
+			return -EINVAL;
+		}
+
+		ace_size = le16_to_cpu(pace->size);
+		if (end_of_dacl - acl_base < ace_size) {
+			cifs_dbg(VFS, "ACL too small to parse ACE\n");
+			return -EINVAL;
+		}
+	}
+
+	return 0;
+}
+
 static void parse_dacl(struct cifs_acl *pdacl, char *end_of_acl,
 		       struct cifs_sid *pownersid, struct cifs_sid *pgrpsid,
 		       struct cifs_fattr *fattr, bool mode_from_special_sid)
@@ -760,7 +832,7 @@ static void parse_dacl(struct cifs_acl *pdacl, char *end_of_acl,
 	int i;
 	int num_aces = 0;
 	int acl_size;
-	char *acl_base;
+	char *acl_base, *end_of_dacl;
 	struct cifs_ace **ppace;
 
 	/* BB need to add parm so we can store the SID BB */
@@ -772,11 +844,8 @@ static void parse_dacl(struct cifs_acl *pdacl, char *end_of_acl,
 		return;
 	}
 
-	/* validate that we do not go past end of acl */
-	if (end_of_acl < (char *)pdacl + le16_to_cpu(pdacl->size)) {
-		cifs_dbg(VFS, "ACL too small to parse DACL\n");
+	if (validate_dacl(pdacl, end_of_acl))
 		return;
-	}
 
 	cifs_dbg(NOISY, "DACL revision %d size %d num aces %d\n",
 		 le16_to_cpu(pdacl->revision), le16_to_cpu(pdacl->size),
@@ -787,6 +856,7 @@ static void parse_dacl(struct cifs_acl *pdacl, char *end_of_acl,
 	   user/group/other have no permissions */
 	fattr->cf_mode &= ~(0777);
 
+	end_of_dacl = (char *)pdacl + le16_to_cpu(pdacl->size);
 	acl_base = (char *)pdacl;
 	acl_size = sizeof(struct cifs_acl);
 
@@ -804,7 +874,7 @@ static void parse_dacl(struct cifs_acl *pdacl, char *end_of_acl,
 		for (i = 0; i < num_aces; ++i) {
 			ppace[i] = (struct cifs_ace *) (acl_base + acl_size);
 #ifdef CONFIG_CIFS_DEBUG2
-			dump_ace(ppace[i], end_of_acl);
+			dump_ace(ppace[i], end_of_dacl);
 #endif
 			if (mode_from_special_sid &&
 			    (compare_sids(&(ppace[i]->sid),
@@ -1262,10 +1332,9 @@ static int build_sec_desc(struct cifs_ntsd *pntsd, struct cifs_ntsd *pnntsd,
 	dacloffset = le32_to_cpu(pntsd->dacloffset);
 	if (dacloffset) {
 		dacl_ptr = (struct cifs_acl *)((char *)pntsd + dacloffset);
-		if (end_of_acl < (char *)dacl_ptr + le16_to_cpu(dacl_ptr->size)) {
-			cifs_dbg(VFS, "Server returned illegal ACL size\n");
-			return -EINVAL;
-		}
+		rc = validate_dacl(dacl_ptr, end_of_acl);
+		if (rc)
+			return rc;
 	}
 
 	owner_sid_ptr = (struct cifs_sid *)((char *)pntsd +
@@ -1625,6 +1694,12 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
 		dacloffset = le32_to_cpu(pntsd->dacloffset);
 		if (dacloffset) {
 			dacl_ptr = (struct cifs_acl *)((char *)pntsd + dacloffset);
+			rc = validate_dacl(dacl_ptr, (char *)pntsd + secdesclen);
+			if (rc) {
+				kfree(pntsd);
+				cifs_put_tlink(tlink);
+				return rc;
+			}
 			if (mode_from_sid)
 				nsecdesclen +=
 					le32_to_cpu(dacl_ptr->num_aces) * sizeof(struct cifs_ace);
-- 
2.53.0