Re: [syzbot] [integrity?] [lsm?] possible deadlock in process_measurement (4)

[Roberto Sassu]

On Mon, 2024-10-07 at 12:58 -0400, Paul Moore wrote:
> On Mon, Oct 7, 2024 at 12:49 PM Roberto Sassu
> <roberto.sassu@xxxxxxxxxxxxxxx> wrote:
> > On Mon, 2024-10-07 at 12:35 -0400, Paul Moore wrote:
> > > On Mon, Oct 7, 2024 at 11:31 AM Roberto Sassu
> > > <roberto.sassu@xxxxxxxxxxxxxxx> wrote:
> > > > On Wed, 2024-10-02 at 23:09 -0400, Paul Moore wrote:
> > > > > On Sat, Sep 28, 2024 at 2:08 PM Shu Han <ebpqwerty472123@xxxxxxxxx> wrote:
> > > > > > 
> > > > > > > ======================================================
> > > > > > > WARNING: possible circular locking dependency detected
> > > > > > > 6.11.0-syzkaller-10045-g97d8894b6f4c #0 Not tainted
> > > > > > > ------------------------------------------------------
> > > > > > > syz-executor369/5231 is trying to acquire lock:
> > > > > > > ffff888072852370 (&sb->s_type->i_mutex_key#12){+.+.}-{3:3}, at: inode_lock include/linux/fs.h:815 [inline]
> > > > > > > ffff888072852370 (&sb->s_type->i_mutex_key#12){+.+.}-{3:3}, at: process_measurement+0x439/0x1fb0 security/integrity/ima/ima_main.c:250
> > > > > > > 
> > > > > > > but task is already holding lock:
> > > > > > > ffff88807ac9a798 (&mm->mmap_lock){++++}-{3:3}, at: mmap_write_lock_killable include/linux/mmap_lock.h:122 [inline]
> > > > > > > ffff88807ac9a798 (&mm->mmap_lock){++++}-{3:3}, at: __do_sys_remap_file_pages mm/mmap.c:1649 [inline]
> > > > > > > ffff88807ac9a798 (&mm->mmap_lock){++++}-{3:3}, at: __se_sys_remap_file_pages+0x22d/0xa50 mm/mmap.c:1624
> > > > > > > 
> > > > > > > which lock already depends on the new lock.
> > > > > > 
> > > > > > This issue (if not a false positive?) is due to the possible `prot`
> > > > > > change caused by the processing logic for READ_IMPLIES_EXEC in do_mmap(),
> > > > > > so the remap_file_pages() must perform LSM check before calling do_mmap(),
> > > > > > this is what the previous commit want to do.
> > > > > 
> > > > > My apologies for the delay on this, I was traveling for a bit and
> > > > > missed this issue while away.
> > > > > 
> > > > > Looking quickly at the report, I don't believe this is a false positive.
> > > > > 
> > > > > > The LSM check is required to know what the `prot` is, but `prot` must be
> > > > > > obtained after holding the `mmap_write_lock`.
> > > > > > 
> > > > > > If the `mmap_write_lock` is released after getting the `prot` and before
> > > > > > the LSM call in remap_file_pages(), it may cause TOCTOU.
> > > > > 
> > > > > Looking at the IMA code, specifically the process_measurement()
> > > > > function which is called from the security_mmap_file() LSM hook, I'm
> > > > > not sure why there is the inode_lock() protected region.  Mimi?
> > > > > Roberto?  My best guess is that locking the inode may have been
> > > > > necessary before we moved the IMA inode state into the inode's LSM
> > > > > security blob, but I'm not certain.
> > > > > 
> > > > > Mimi and Roberto, can we safely remove the inode locking in
> > > > > process_measurement()?
> > > > 
> > > > I discussed a bit with Mimi. Her concern was the duplicate iint
> > > > structure creation during concurrent file accesses. Now that inode
> > > > integrity metadata have been moved to the inode security blob, we can
> > > > take the iint->mutex out of the ima_iint_cache structure, and store it
> > > > directly in the security blob. In this way, we can remove the inode
> > > > lock.
> > > > 
> > > > Will write a patch and see if it passes our tests.
> > > 
> > > That's great, thanks Roberto.  Assuming all goes well we'll want to
> > > backport this everywhere we merged the remap_file_pages() patch.
> > 
> > Welcome. Probably it can go down only until the kernel where IMA and
> > EVM are LSMs.
> 
> Yes, we'll need to look at that once we solve this in Linus' tree.

#syz test: https://github.com/robertosassu/linux.git ima-remove-inode-lock-v1