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audit,io_uring: io_uring openat triggers audit reference count underflow

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An io_uring openat operation can update an audit reference count
from multiple threads resulting in the call trace below.

A call to io_uring_submit() with a single openat op with a flag of
IOSQE_ASYNC results in the following reference count updates.

These first part of the system call performs two increments that do not race.

do_syscall_64()
  __do_sys_io_uring_enter()
    io_submit_sqes()
      io_openat_prep()
        __io_openat_prep()
          getname()
            getname_flags()       /* update 1 (increment) */
              __audit_getname()   /* update 2 (increment) */

The openat op is queued to an io_uring worker thread which starts the
opportunity for a race.  The system call exit performs one decrement.

do_syscall_64()
  syscall_exit_to_user_mode()
    syscall_exit_to_user_mode_prepare()
      __audit_syscall_exit()
        audit_reset_context()
           putname()              /* update 3 (decrement) */

The io_uring worker thread performs one increment and two decrements.
These updates can race with the system call decrement.

io_wqe_worker()
  io_worker_handle_work()
    io_wq_submit_work()
      io_issue_sqe()
        io_openat()
          io_openat2()
            do_filp_open()
              path_openat()
                __audit_inode()   /* update 4 (increment) */
            putname()             /* update 5 (decrement) */
        __audit_uring_exit()
          audit_reset_context()
            putname()             /* update 6 (decrement) */

The fix is to change the refcnt member of struct audit_names
from int to atomic_t.

kernel BUG at fs/namei.c:262!
Call Trace:
...
 ? putname+0x68/0x70
 audit_reset_context.part.0.constprop.0+0xe1/0x300
 __audit_uring_exit+0xda/0x1c0
 io_issue_sqe+0x1f3/0x450
 ? lock_timer_base+0x3b/0xd0
 io_wq_submit_work+0x8d/0x2b0
 ? __try_to_del_timer_sync+0x67/0xa0
 io_worker_handle_work+0x17c/0x2b0
 io_wqe_worker+0x10a/0x350

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/lkml/MW2PR2101MB1033FFF044A258F84AEAA584F1C9A@MW2PR2101MB1033.namprd21.prod.outlook.com/
Fixes: 5bd2182d58 ("audit,io_uring,io-wq: add some basic audit support to io_uring")
Signed-off-by: Dan Clash <daclash@linux.microsoft.com>
Link: https://lore.kernel.org/r/20231012215518.GA4048@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Dan Clash 2023-10-12 14:55:18 -07:00 committed by Christian Brauner
parent 94f6f0550c
commit 03adc61eda
No known key found for this signature in database
GPG key ID: 91C61BC06578DCA2
3 changed files with 10 additions and 9 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

9
fs/namei.c
View file

@ -188,7 +188,7 @@ getname_flags(const char __user *filename, int flags, int *empty)
}
}
result->refcnt = 1;
atomic_set(&result->refcnt, 1);
/* The empty path is special. */
if (unlikely(!len)) {
if (empty)
@ -249,7 +249,7 @@ getname_kernel(const char * filename)
memcpy((char *)result->name, filename, len);
result->uptr = NULL;
result->aname = NULL;
result->refcnt = 1;
atomic_set(&result->refcnt, 1);
audit_getname(result);
return result;
@ -261,9 +261,10 @@ void putname(struct filename *name)
if (IS_ERR(name))
return;
BUG_ON(name->refcnt <= 0);
if (WARN_ON_ONCE(!atomic_read(&name->refcnt)))
return;
if (--name->refcnt > 0)
if (!atomic_dec_and_test(&name->refcnt))
return;
if (name->name != name->iname) {

2
include/linux/fs.h
View file

@ -2403,7 +2403,7 @@ struct audit_names;
struct filename {
const char *name; /* pointer to actual string */
const __user char *uptr; /* original userland pointer */
int refcnt;
atomic_t refcnt;
struct audit_names *aname;
const char iname[];
};

8
kernel/auditsc.c
View file

@ -2212,7 +2212,7 @@ __audit_reusename(const __user char *uptr)
if (!n->name)
continue;
if (n->name->uptr == uptr) {
n->name->refcnt++;
atomic_inc(&n->name->refcnt);
return n->name;
}
}
@ -2241,7 +2241,7 @@ void __audit_getname(struct filename *name)
n->name = name;
n->name_len = AUDIT_NAME_FULL;
name->aname = n;
name->refcnt++;
atomic_inc(&name->refcnt);
}
static inline int audit_copy_fcaps(struct audit_names *name,
@ -2373,7 +2373,7 @@ void __audit_inode(struct filename *name, const struct dentry *dentry,
return;
if (name) {
n->name = name;
name->refcnt++;
atomic_inc(&name->refcnt);
}
out:
@ -2500,7 +2500,7 @@ void __audit_inode_child(struct inode *parent,
if (found_parent) {
found_child->name = found_parent->name;
found_child->name_len = AUDIT_NAME_FULL;
found_child->name->refcnt++;
atomic_inc(&found_child->name->refcnt);
}
}