Re: [PATCH] smb: client: fix DACL-rewrite heap overflow in id_mode_to_cifs_acl()

Hi Steve,

On Thu, Jul 09, 2026 at 05:15:40PM -0500, Steve French wrote:
> When I tried this it changed the length used (for chown with cifsacl
> mount option) from 88 bytes to 236 bytes
> which seems suspicious.  Have you been able to reproduce the bug this
> patch is supposed to fix?

I shared my reproducer script via PM.

This triggered the following KASAN splat in testing (and no longer does
with the patch applied):

[   46.251162] ==================================================================
[   46.278880] BUG: KASAN: slab-out-of-bounds in build_sec_desc.constprop.0+0x3010/0x3a70 [cifs]
[   46.319912] Write of size 4 at addr ffff888155556374 by task chown/3439
[   46.351546]
[   46.358846] CPU: 4 UID: 0 PID: 3439 Comm: chown Not tainted 7.1.0-rc6+ #35 PREEMPT(full)
[   46.358850] Hardware name: Amazon EC2 c6i.4xlarge/, BIOS 1.0 10/16/2017
[   46.358853] Call Trace:
[   46.358856]  <TASK>
[   46.358859]  dump_stack_lvl+0x51/0x70
[   46.358866]  print_address_description.constprop.0+0x2c/0x3a0
[   46.358871]  ? build_sec_desc.constprop.0+0x3010/0x3a70 [cifs]
[   46.358941]  print_report+0xb4/0x270
[   46.358944]  ? kasan_addr_to_slab+0x9/0x70
[   46.358948]  kasan_report+0xb4/0xe0
[   46.358951]  ? build_sec_desc.constprop.0+0x3010/0x3a70 [cifs]
[   46.359015]  build_sec_desc.constprop.0+0x3010/0x3a70 [cifs]
[   46.359077]  ? queue_folios_pte_range+0x45c/0x7a0
[   46.359082]  ? __pfx_build_sec_desc.constprop.0+0x10/0x10 [cifs]
[   46.359143]  ? __find_readable_file+0x310/0x540 [cifs]
[   46.359220]  ? kasan_save_track+0x10/0x30
[   46.359222]  ? __kasan_kmalloc+0x7b/0x90
[   46.359226]  id_mode_to_cifs_acl+0x31c/0x760 [cifs]
[   46.359295]  ? __pfx_id_mode_to_cifs_acl+0x10/0x10 [cifs]
[   46.359356]  ? __build_path_from_dentry_optional_prefix+0x176/0x620 [cifs]
[   46.359432]  cifs_setattr_nounix+0xc5b/0x1860 [cifs]
[   46.359505]  ? __pfx_cifs_setattr_nounix+0x10/0x10 [cifs]
[   46.359573]  ? __pfx___vfs_getxattr+0x10/0x10
[   46.359577]  ? __pfx_current_time+0x10/0x10
[   46.359580]  cifs_setattr+0x173/0x2b0 [cifs]
[   46.359648]  notify_change+0x832/0xf20
[   46.359651]  ? __pfx_from_vfsuid+0x10/0x10
[   46.359655]  ? chown_common+0x422/0x5e0
[   46.359658]  chown_common+0x422/0x5e0
[   46.359662]  ? __pfx_chown_common+0x10/0x10
[   46.359665]  ? check_heap_object+0x6f/0x490
[   46.359669]  ? strncpy_from_user+0x3b/0x1f0
[   46.359673]  do_fchownat+0x124/0x160
[   46.359677]  ? __pfx_do_fchownat+0x10/0x10
[   46.359680]  __x64_sys_fchownat+0xb9/0x150
[   46.359684]  ? arch_exit_to_user_mode_prepare.constprop.0+0x95/0xc0
[   46.359688]  do_syscall_64+0xaf/0x550
[   46.359693]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[   46.359695] RIP: 0033:0x7fd413b00b0e
[   46.359709] Code: 48 8b 0d ed b2 0f 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 49 89 ca b8 04 01 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d ba b2 0f 00 f7 d8 64 89 01 48
[   46.359712] RSP: 002b:00007ffc3eb973d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000104
[   46.359715] RAX: ffffffffffffffda RBX: 00007ffc3eb97700 RCX: 00007fd413b00b0e
[   46.359717] RDX: 0000000000001770 RSI: 000055e6afa81740 RDI: 00000000ffffff9c
[   46.359719] RBP: 000055e6afa7fb70 R08: 0000000000000000 R09: 0000000000000000
[   46.359720] R10: 00000000ffffffff R11: 0000000000000246 R12: 000055e6afa81740
[   46.359722] R13: 0000000000000001 R14: 00000000ffffff9c R15: 000055e6afa7fb00
[   46.359725]  </TASK>
[   46.359726]
[   47.532861] Allocated by task 3439:
[   47.549557]  kasan_save_stack+0x20/0x40
[   47.567957]  kasan_save_track+0x10/0x30
[   47.586353]  __kasan_kmalloc+0x7b/0x90
[   47.604316]  __kmalloc_noprof+0x1c1/0x530
[   47.623566]  id_mode_to_cifs_acl+0x2d0/0x760 [cifs]
[   47.646748]  cifs_setattr_nounix+0xc5b/0x1860 [cifs]
[   47.670768]  cifs_setattr+0x173/0x2b0 [cifs]
[   47.690945]  notify_change+0x832/0xf20
[   47.708918]  chown_common+0x422/0x5e0
[   47.726468]  do_fchownat+0x124/0x160
[   47.743582]  __x64_sys_fchownat+0xb9/0x150
[   47.763261]  do_syscall_64+0xaf/0x550
[   47.780816]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[   47.804785]
[   47.812163] The buggy address belongs to the object at ffff888155556000
[   47.812163]  which belongs to the cache kmalloc-1k of size 1024
[   47.872006] The buggy address is located 0 bytes to the right of
[   47.872006]  allocated 884-byte region [ffff888155556000, ffff888155556374)
[   47.934359]
[   47.941237] The buggy address belongs to the physical page:
[   47.967735] page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x155550
[   48.006191] head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
[   48.042968] flags: 0x17ffffc0000040(head|node=0|zone=2|lastcpupid=0x1fffff)
[   48.076404] page_type: f5(slab)
[   48.091399] raw: 0017ffffc0000040 ffff888100042dc0 dead000000000100 dead000000000122
[   48.128595] raw: 0000000000000000 0000000800100010 00000000f5000000 0000000000000000
[   48.165814] head: 0017ffffc0000040 ffff888100042dc0 dead000000000100 dead000000000122
[   48.203005] head: 0000000000000000 0000000800100010 00000000f5000000 0000000000000000
[   48.240615] head: 0017ffffc0000003 fffffffffffffe01 00000000ffffffff 00000000ffffffff
[   48.278257] head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008
[   48.315854] page dumped because: kasan: bad access detected
[   48.342361]
[   48.349658] Memory state around the buggy address:
[   48.372332]  ffff888155556200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[   48.407026]  ffff888155556280: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[   48.441656] >ffff888155556300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 fc
[   48.476272]                                                              ^
[   48.509238]  ffff888155556380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[   48.543873]  ffff888155556400: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[   48.578508] ==================================================================


Bjoern

> On Thu, Jul 9, 2026 at 10:55 AM Bjoern Doebel <[email protected]> wrote:
> >
> > Budget the destination buffer for the worst case in both branches:
> > every rewritten ACE may take sizeof(struct smb_ace) bytes (which
> > already accounts for an smb_sid with SID_MAX_SUB_AUTHORITIES
> > sub-authorities), plus the smb_acl header that
> > replace_sids_and_copy_aces() emits.
> >
> > Fixes: bc3e9dd9d104 ("cifs: Change SIDs in ACEs while transferring file ownership.")
> > Cc: [email protected]
> > Signed-off-by: Bjoern Doebel <[email protected]>
> > Assisted-by: Kiro:claude-opus-4.6
> > ---
> >  fs/smb/client/cifsacl.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/smb/client/cifsacl.c b/fs/smb/client/cifsacl.c
> > index 07cf0e5782337..6d572dd995d79 100644
> > --- a/fs/smb/client/cifsacl.c
> > +++ b/fs/smb/client/cifsacl.c
> > @@ -1812,11 +1812,13 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
> >                                 cifs_put_tlink(tlink);
> >                                 return rc;
> >                         }
> > -                       if (mode_from_sid)
> > -                               nsecdesclen +=
> > -                                       le16_to_cpu(dacl_ptr->num_aces) * sizeof(struct smb_ace);
> > -                       else /* cifsacl */
> > -                               nsecdesclen += le16_to_cpu(dacl_ptr->size);
> > +                       /*
> > +                        * Worst case: every ACE is rewritten with a new SID of
> > +                        * SID_MAX_SUB_AUTHORITIES sub-auths -> sizeof(smb_ace) each,
> > +                        * plus the smb_acl header replace_sids_and_copy_aces() emits.
> > +                        */
> > +                       nsecdesclen += sizeof(struct smb_acl) +
> > +                               le16_to_cpu(dacl_ptr->num_aces) * sizeof(struct smb_ace);
> >                 }
> >         }
> >
> > --
> > 2.50.1
> >
> >
> 
> 
> -- 
> Thanks,
> 
> Steve