[PATCH] lsm: don't call d_path() when current->fs is NULL

[PATCH] lsm: don't call d_path() when current->fs is NULL

[Drif Abdelmalek Mohamed Said]

When the last reference to a file is put from exit_task_work(), the task
has already run exit_fs(), so current->fs is NULL.

syzbot reported:

Unable to handle kernel paging request at virtual address dfff800000000002
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]

Call trace:
 kasan_mem_to_shadow include/linux/kasan.h:65 [inline] (P)
 kasan_byte_accessible+0xc/0x1c mm/kasan/generic.c:210 (P)
 kasan_check_byte include/linux/kasan.h:402 [inline]
 lock_acquire+0xa4/0x320 kernel/locking/lockdep.c:5916
 seqcount_lockdep_reader_access include/linux/seqlock.h:73 [inline]
 read_seqbegin+0x6c/0x144 include/linux/seqlock.h:838
 get_fs_root_rcu fs/d_path.c:244 [inline]
 d_path+0x224/0x4dc fs/d_path.c:287
 audit_log_d_path+0xdc/0x19c kernel/audit.c:2221
 audit_log_lsm_data+0xe0/0x12d0 security/lsm_audit.c:199
 dump_common_audit_data security/lsm_audit.c:421 [inline]
 common_lsm_audit+0x1d0/0x290 security/lsm_audit.c:451
 smack_log+0x1f4/0x264 security/smack/smack_access.c:400
 smk_tskacc+0x210/0x2ac security/smack/smack_access.c:255
 smack_file_open+0x1d0/0x230 security/smack/smack_lsm.c:2078
 security_file_open+0xec/0x258 security/security.c:2739
 do_dentry_open+0x2a8/0xfbc fs/open.c:973
 vfs_open+0x44/0x2dc fs/open.c:1101
 dentry_open+0x74/0xc0 fs/open.c:1124
 ima_calc_file_hash+0x120/0x5cc security/integrity/ima/ima_crypto.c:269
 ima_collect_measurement+0x420/0x874 security/integrity/ima/ima_api.c:300
 ima_update_xattr+0x160/0x230 security/integrity/ima/ima_appraise.c:617
 ima_check_last_writer security/integrity/ima/ima_main.c:227 [inline]
 ima_file_free+0x40c/0x528 security/integrity/ima/ima_main.c:251
 security_file_release+0x84/0x148 security/security.c:2424
 __fput+0x2c4/0x74c fs/file_table.c:506
 ____fput+0x20/0x30 fs/file_table.c:541
 task_work_run+0x1cc/0x25c kernel/task_work.c:233
 exit_task_work include/linux/task_work.h:40 [inline]
 do_exit+0x58c/0x1a74 kernel/exit.c:986
 do_group_exit+0x198/0x238 kernel/exit.c:1129
 __do_sys_exit_group kernel/exit.c:1140 [inline]
 __se_sys_exit_group kernel/exit.c:1138 [inline]
 pid_child_should_wake+0x0/0x104 kernel/exit.c:1138
 __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
 invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
 el0_svc_common+0x19c/0x23c arch/arm64/kernel/syscall.c:121
 do_el0_svc+0x4c/0x5c arch/arm64/kernel/syscall.c:140
 el0_svc+0x60/0x1d8 arch/arm64/kernel/entry-common.c:796
 el0t_64_sync_handler+0x44/0x104 arch/arm64/kernel/entry-common.c:815
 el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:606

d_path() renders a path relative to current->fs->root.  After exit_fs()
there is no such root, and no substitute root is meaningful: the
record's pid/comm describe a task that can no longer name the file, and
the file may have been opened by a different task entirely.  Falling
back to another root would produce a well-formed but misleading path,
so report the field as unknown instead, as exe=(null) and name=(null)
already do.

Handle this in audit_log_lsm_data(), where the meaning of the field is
known, for LSM_AUDIT_DATA_PATH (Smack file_open), LSM_AUDIT_DATA_FILE
(SELinux file_open), LSM_AUDIT_DATA_IOCTL_OP and the AF_UNIX path in
LSM_AUDIT_DATA_NET.

Alternative: omit " path=" as it is done for inode in audit_log_lsm_data.

Fixes: 4a9d4b024a31 ("switch fput to task_work_add")
Reported-by: syzbot+975ace299d6ba2eb1cea@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=975ace299d6ba2eb1cea
Signed-off-by: Drif Abdelmalek Mohamed Said <drifabdelmalekmohamedsaid@gmail.com>
---
 security/lsm_audit.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/security/lsm_audit.c b/security/lsm_audit.c
index 955b2e7b2c8e..5ed505d781bb 100644
--- a/security/lsm_audit.c
+++ b/security/lsm_audit.c
@@ -169,6 +169,15 @@ static inline void print_ipv4_addr(struct audit_buffer *ab, __be32 addr,
 		audit_log_format(ab, " %s=%d", name2, ntohs(port));
 }
 
+static inline void audit_log_lsm_path(struct audit_buffer *ab,
+									const struct path *path)
+{
+	if (current->fs)
+		audit_log_d_path(ab, " path=", path);
+	else
+		audit_log_format(ab, " path=(null)");
+}
+
 /**
  * audit_log_lsm_data - helper to log common LSM audit data
  * @ab : the audit buffer
@@ -196,7 +205,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
 	case LSM_AUDIT_DATA_PATH: {
 		struct inode *inode;
 
-		audit_log_d_path(ab, " path=", &a->u.path);
+		audit_log_lsm_path(ab, &a->u.path);
 
 		inode = d_backing_inode(a->u.path.dentry);
 		if (inode) {
@@ -209,7 +218,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
 	case LSM_AUDIT_DATA_FILE: {
 		struct inode *inode;
 
-		audit_log_d_path(ab, " path=", &a->u.file->f_path);
+		audit_log_lsm_path(ab, &a->u.file->f_path);
 
 		inode = file_inode(a->u.file);
 		if (inode) {
@@ -222,7 +231,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
 	case LSM_AUDIT_DATA_IOCTL_OP: {
 		struct inode *inode;
 
-		audit_log_d_path(ab, " path=", &a->u.op->path);
+		audit_log_lsm_path(ab, &a->u.op->path);
 
 		inode = a->u.op->path.dentry->d_inode;
 		if (inode) {
@@ -321,7 +330,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
 				if (!addr)
 					break;
 				if (u->path.dentry) {
-					audit_log_d_path(ab, " path=", &u->path);
+					audit_log_lsm_path(ab, &u->path);
 					break;
 				}
 				len = addr->len-sizeof(short);
-- 
2.43.0

Re: [PATCH] lsm: don't call d_path() when current->fs is NULL

[DRIF ABDELMALEK MOHAMED SAID]

Hello,
Just following up on this patch to see if anyone has had a chance to
review it or if any changes are needed since we're at 7.4-rc1.
Thanks,
Drif Abdelmalek Mohamed Said

On Mon, Oct 19, 2026 at 11:29 PM Drif Abdelmalek Mohamed Said
<drifabdelmalekmohamedsaid@gmail.com> wrote:
>
> When the last reference to a file is put from exit_task_work(), the task
> has already run exit_fs(), so current->fs is NULL.
>
> syzbot reported:
>
> Unable to handle kernel paging request at virtual address dfff800000000002
> KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
>
> Call trace:
>  kasan_mem_to_shadow include/linux/kasan.h:65 [inline] (P)
>  kasan_byte_accessible+0xc/0x1c mm/kasan/generic.c:210 (P)
>  kasan_check_byte include/linux/kasan.h:402 [inline]
>  lock_acquire+0xa4/0x320 kernel/locking/lockdep.c:5916
>  seqcount_lockdep_reader_access include/linux/seqlock.h:73 [inline]
>  read_seqbegin+0x6c/0x144 include/linux/seqlock.h:838
>  get_fs_root_rcu fs/d_path.c:244 [inline]
>  d_path+0x224/0x4dc fs/d_path.c:287
>  audit_log_d_path+0xdc/0x19c kernel/audit.c:2221
>  audit_log_lsm_data+0xe0/0x12d0 security/lsm_audit.c:199
>  dump_common_audit_data security/lsm_audit.c:421 [inline]
>  common_lsm_audit+0x1d0/0x290 security/lsm_audit.c:451
>  smack_log+0x1f4/0x264 security/smack/smack_access.c:400
>  smk_tskacc+0x210/0x2ac security/smack/smack_access.c:255
>  smack_file_open+0x1d0/0x230 security/smack/smack_lsm.c:2078
>  security_file_open+0xec/0x258 security/security.c:2739
>  do_dentry_open+0x2a8/0xfbc fs/open.c:973
>  vfs_open+0x44/0x2dc fs/open.c:1101
>  dentry_open+0x74/0xc0 fs/open.c:1124
>  ima_calc_file_hash+0x120/0x5cc security/integrity/ima/ima_crypto.c:269
>  ima_collect_measurement+0x420/0x874 security/integrity/ima/ima_api.c:300
>  ima_update_xattr+0x160/0x230 security/integrity/ima/ima_appraise.c:617
>  ima_check_last_writer security/integrity/ima/ima_main.c:227 [inline]
>  ima_file_free+0x40c/0x528 security/integrity/ima/ima_main.c:251
>  security_file_release+0x84/0x148 security/security.c:2424
>  __fput+0x2c4/0x74c fs/file_table.c:506
>  ____fput+0x20/0x30 fs/file_table.c:541
>  task_work_run+0x1cc/0x25c kernel/task_work.c:233
>  exit_task_work include/linux/task_work.h:40 [inline]
>  do_exit+0x58c/0x1a74 kernel/exit.c:986
>  do_group_exit+0x198/0x238 kernel/exit.c:1129
>  __do_sys_exit_group kernel/exit.c:1140 [inline]
>  __se_sys_exit_group kernel/exit.c:1138 [inline]
>  pid_child_should_wake+0x0/0x104 kernel/exit.c:1138
>  __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
>  invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
>  el0_svc_common+0x19c/0x23c arch/arm64/kernel/syscall.c:121
>  do_el0_svc+0x4c/0x5c arch/arm64/kernel/syscall.c:140
>  el0_svc+0x60/0x1d8 arch/arm64/kernel/entry-common.c:796
>  el0t_64_sync_handler+0x44/0x104 arch/arm64/kernel/entry-common.c:815
>  el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:606
>
> d_path() renders a path relative to current->fs->root.  After exit_fs()
> there is no such root, and no substitute root is meaningful: the
> record's pid/comm describe a task that can no longer name the file, and
> the file may have been opened by a different task entirely.  Falling
> back to another root would produce a well-formed but misleading path,
> so report the field as unknown instead, as exe=(null) and name=(null)
> already do.
>
> Handle this in audit_log_lsm_data(), where the meaning of the field is
> known, for LSM_AUDIT_DATA_PATH (Smack file_open), LSM_AUDIT_DATA_FILE
> (SELinux file_open), LSM_AUDIT_DATA_IOCTL_OP and the AF_UNIX path in
> LSM_AUDIT_DATA_NET.
>
> Alternative: omit " path=" as it is done for inode in audit_log_lsm_data.
>
> Fixes: 4a9d4b024a31 ("switch fput to task_work_add")
> Reported-by: syzbot+975ace299d6ba2eb1cea@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=975ace299d6ba2eb1cea
> Signed-off-by: Drif Abdelmalek Mohamed Said <drifabdelmalekmohamedsaid@gmail.com>
> ---
>  security/lsm_audit.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/security/lsm_audit.c b/security/lsm_audit.c
> index 955b2e7b2c8e..5ed505d781bb 100644
> --- a/security/lsm_audit.c
> +++ b/security/lsm_audit.c
> @@ -169,6 +169,15 @@ static inline void print_ipv4_addr(struct audit_buffer *ab, __be32 addr,
>                 audit_log_format(ab, " %s=%d", name2, ntohs(port));
>  }
>
> +static inline void audit_log_lsm_path(struct audit_buffer *ab,
> +                                                                       const struct path *path)
> +{
> +       if (current->fs)
> +               audit_log_d_path(ab, " path=", path);
> +       else
> +               audit_log_format(ab, " path=(null)");
> +}
> +
>  /**
>   * audit_log_lsm_data - helper to log common LSM audit data
>   * @ab : the audit buffer
> @@ -196,7 +205,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
>         case LSM_AUDIT_DATA_PATH: {
>                 struct inode *inode;
>
> -               audit_log_d_path(ab, " path=", &a->u.path);
> +               audit_log_lsm_path(ab, &a->u.path);
>
>                 inode = d_backing_inode(a->u.path.dentry);
>                 if (inode) {
> @@ -209,7 +218,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
>         case LSM_AUDIT_DATA_FILE: {
>                 struct inode *inode;
>
> -               audit_log_d_path(ab, " path=", &a->u.file->f_path);
> +               audit_log_lsm_path(ab, &a->u.file->f_path);
>
>                 inode = file_inode(a->u.file);
>                 if (inode) {
> @@ -222,7 +231,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
>         case LSM_AUDIT_DATA_IOCTL_OP: {
>                 struct inode *inode;
>
> -               audit_log_d_path(ab, " path=", &a->u.op->path);
> +               audit_log_lsm_path(ab, &a->u.op->path);
>
>                 inode = a->u.op->path.dentry->d_inode;
>                 if (inode) {
> @@ -321,7 +330,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
>                                 if (!addr)
>                                         break;
>                                 if (u->path.dentry) {
> -                                       audit_log_d_path(ab, " path=", &u->path);
> +                                       audit_log_lsm_path(ab, &u->path);
>                                         break;
>                                 }
>                                 len = addr->len-sizeof(short);
> --
> 2.43.0
>

Re: [PATCH] lsm: don't call d_path() when current->fs is NULL

[Paul Moore]

On Tue, Nov 3, 2026 at 3:18 PM DRIF ABDELMALEK MOHAMED SAID
<drifabdelmalekmohamedsaid@gmail.com> wrote:
>
> Hello,
> Just following up on this patch to see if anyone has had a chance to
> review it or if any changes are needed since we're at 7.4-rc1.
> Thanks,
> Drif Abdelmalek Mohamed Said

[CC'ing the audit list]

Not yet, but it is in my queue.  Regardless, thank you for the report
and the patch!

-- 
paul-moore.com

Re: [PATCH] lsm: don't call d_path() when current->fs is NULL

[Paul Moore]

On Oct 19, 2026 DRIF ABDELMALEK MOHAMED SAID <drifabdelmalekmohamedsaid@gmail.com> wrote:
> 
> When the last reference to a file is put from exit_task_work(), the task
> has already run exit_fs(), so current->fs is NULL.
> 
> syzbot reported:
> 
> Unable to handle kernel paging request at virtual address dfff800000000002
> KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
> 
> Call trace:
>  kasan_mem_to_shadow include/linux/kasan.h:65 [inline] (P)
>  kasan_byte_accessible+0xc/0x1c mm/kasan/generic.c:210 (P)
>  kasan_check_byte include/linux/kasan.h:402 [inline]
>  lock_acquire+0xa4/0x320 kernel/locking/lockdep.c:5916
>  seqcount_lockdep_reader_access include/linux/seqlock.h:73 [inline]
>  read_seqbegin+0x6c/0x144 include/linux/seqlock.h:838
>  get_fs_root_rcu fs/d_path.c:244 [inline]
>  d_path+0x224/0x4dc fs/d_path.c:287
>  audit_log_d_path+0xdc/0x19c kernel/audit.c:2221
>  audit_log_lsm_data+0xe0/0x12d0 security/lsm_audit.c:199

...

> d_path() renders a path relative to current->fs->root.  After exit_fs()
> there is no such root, and no substitute root is meaningful: the
> record's pid/comm describe a task that can no longer name the file, and
> the file may have been opened by a different task entirely.  Falling
> back to another root would produce a well-formed but misleading path,
> so report the field as unknown instead, as exe=(null) and name=(null)
> already do.
> 
> Handle this in audit_log_lsm_data(), where the meaning of the field is
> known, for LSM_AUDIT_DATA_PATH (Smack file_open), LSM_AUDIT_DATA_FILE
> (SELinux file_open), LSM_AUDIT_DATA_IOCTL_OP and the AF_UNIX path in
> LSM_AUDIT_DATA_NET.
> 
> Alternative: omit " path=" as it is done for inode in audit_log_lsm_data.

Once again, thanks for reporting this and providing a fix!

More on this below, but I think the idea that in the !current->fs case we
can follow the LSM_AUDIT_DATA_INODE example and log something useful, at
least more useful than "path=(null)" is a good one.

> Fixes: 4a9d4b024a31 ("switch fput to task_work_add")
> Reported-by: syzbot+975ace299d6ba2eb1cea@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=975ace299d6ba2eb1cea
> Signed-off-by: Drif Abdelmalek Mohamed Said <drifabdelmalekmohamedsaid@gmail.com>
> ---
>  security/lsm_audit.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/security/lsm_audit.c b/security/lsm_audit.c
> index 955b2e7b2c8e..5ed505d781bb 100644
> --- a/security/lsm_audit.c
> +++ b/security/lsm_audit.c
> @@ -169,6 +169,15 @@ static inline void print_ipv4_addr(struct audit_buffer *ab, __be32 addr,
>  		audit_log_format(ab, " %s=%d", name2, ntohs(port));
>  }
>  
> +static inline void audit_log_lsm_path(struct audit_buffer *ab,
> +									const struct path *path)

The horizontal spacing above looks ... odd.  Can you please fix this so
follows the established kernel style by aligning it with the parameter
list in the line above?

> +{
> +	if (current->fs)
> +		audit_log_d_path(ab, " path=", path);
> +	else
> +		audit_log_format(ab, " path=(null)");

What if we were to extract the LSM_AUDIT_DATA_INODE logging into a
helper function, similar to what you've done here, and call it when
current->fs is NULL?  What do you think?

I would also suggest we use "path=?" in the case when we can't resolve
the path or do any other meaningful logging; this is more consistent
with existing audit approaches than "path=(null)".

  static void audit_log_lsm_inode(ab, inode)
  {
    ...
  }

  static void audit_log_lsm_path(ab, inode, path)
  {
    if (current->fs)
      audit_log_d_path(...);
    else if (inode)
      audit_log_lsm_inode(ab, inode)
    else
      audit_log_format(ab, " path=?");
  }

> +}
> +
>  /**
>   * audit_log_lsm_data - helper to log common LSM audit data
>   * @ab : the audit buffer
> @@ -196,7 +205,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
>  	case LSM_AUDIT_DATA_PATH: {
>  		struct inode *inode;
>  
> -		audit_log_d_path(ab, " path=", &a->u.path);
> +		audit_log_lsm_path(ab, &a->u.path);
>  
>  		inode = d_backing_inode(a->u.path.dentry);

We would need to move the call down to here, but that should be okay:

  audit_log_lsm_path(ab, inode, &a->u.path);

>  		if (inode) {
> @@ -209,7 +218,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
>  	case LSM_AUDIT_DATA_FILE: {
>  		struct inode *inode;
>  
> -		audit_log_d_path(ab, " path=", &a->u.file->f_path);
> +		audit_log_lsm_path(ab, &a->u.file->f_path);
>  
>  		inode = file_inode(a->u.file);

Similar to the above:

  audit_log_lsm_path(ab, inode, &a->u.file->f_path);

>  		if (inode) {
> @@ -222,7 +231,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
>  	case LSM_AUDIT_DATA_IOCTL_OP: {
>  		struct inode *inode;
>  
> -		audit_log_d_path(ab, " path=", &a->u.op->path);
> +		audit_log_lsm_path(ab, &a->u.op->path);
>  
>  		inode = a->u.op->path.dentry->d_inode;

... and here:

  audit_log_lsm_path(ab, inode, &a->u.op->path);

>  		if (inode) {
> @@ -321,7 +330,7 @@ void audit_log_lsm_data(struct audit_buffer *ab,
>  				if (!addr)
>  					break;
>  				if (u->path.dentry) {
> -					audit_log_d_path(ab, " path=", &u->path);
> +					audit_log_lsm_path(ab, &u->path);

It's probably not very useful to have the inode info for a AF_UNIX
socket, so we can probably just do this:

  audit_log_lsm_path(ab, NULL, &u->path);

>  					break;
>  				}
>  				len = addr->len-sizeof(short);
> -- 
> 2.43.0

--
paul-moore.com