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smb: client: bound dirent name against end of SMB response in cifs_filldir

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cifs_filldir() copies the entry name out of an SMB1 TRANS2_FIND_FIRST /
FIND_NEXT response using a length (de.namelen) supplied by the server.
The kmalloc'd SMB response buffer is bounded, but nothing checks that
de.name + de.namelen still lies inside that buffer before the eventual
filldir64() -> verify_dirent_name() -> memchr() reads namelen bytes.

A hostile SMB1 server that returns an oversized FileNameLength in a
directory entry therefore causes memchr() to read past the end of the
response slab buffer. Reachable from any user who can list a directory
on a CIFS mount served by an attacker-controlled server (getdents64()
on the mounted directory):

  BUG: KASAN: slab-out-of-bounds in memchr+0x71/0x80
  Read of size 1 at addr ffff88800e0640cc by task poc/115
  Call Trace:
   dump_stack_lvl+0x64/0x80
   print_report+0xce/0x620
   kasan_report+0xec/0x120
   memchr+0x71/0x80
   filldir64+0x4c/0x6a0
   cifs_filldir.constprop.0+0x9bb/0x1e00
   cifs_readdir+0x2101/0x3380
   iterate_dir+0x19c/0x520
   __x64_sys_getdents64+0x126/0x210
   do_syscall_64+0x107/0x5a0
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Pass the end-of-response pointer down to cifs_filldir() and reject
entries whose name would extend past that boundary.

This bug was discovered by Artiphishell's vTriage pipeline, which
generated a userspace reproducer (an emulated hostile SMB1 server plus
a getdents64() client) that reliably triggers the KASAN report on an
unpatched kernel. The fix below was drafted with the Claude coding
assistant; a userspace reproducer is available on request.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Jay Vadayath <jay@artiphishell.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
This commit is contained in:
Jay Vadayath 2026-07-17 17:21:22 -07:00 committed by Steve French
parent f6f5ee2aa3
commit f8cf09a53a
1 changed files with 7 additions and 2 deletions
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9
fs/smb/client/readdir.c
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@ -952,7 +952,7 @@ static bool cifs_dir_emit(struct dir_context *ctx,
static int cifs_filldir(char *find_entry, struct file *file,
struct dir_context *ctx,
char *scratch_buf, unsigned int max_len,
struct cached_fid *cfid)
char *end_of_smb, struct cached_fid *cfid)
{
struct cifsFileInfo *file_info = file->private_data;
struct super_block *sb = file_inode(file)->i_sb;
@ -974,6 +974,11 @@ static int cifs_filldir(char *find_entry, struct file *file,
return -EINVAL;
}
if (de.name + de.namelen > end_of_smb) {
cifs_dbg(VFS, "search entry name extends past end of SMB\n");
return -EINVAL;
}
/* skip . and .. since we added them first */
if (cifs_entry_is_dot(&de, file_info->srch_inf.unicode))
return 0;
@ -1194,7 +1199,7 @@ int cifs_readdir(struct file *file, struct dir_context *ctx)
*/
*tmp_buf = 0;
rc = cifs_filldir(current_entry, file, ctx,
tmp_buf, max_len, cfid);
tmp_buf, max_len, end_of_smb, cfid);
if (rc) {
if (rc > 0)
rc = 0;