Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

[Aleksa Sarai]

On 2019-12-29, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sun, Dec 29, 2019 at 11:30 PM Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
> >
> >     BUG: kernel NULL pointer dereference, address: 0000000000000000
> 
> Would you mind building with debug info, and then running the oops through
> 
>  scripts/decode_stacktrace.sh
> 
> which makes those addresses much more legible.

Will do.

> >     #PF: supervisor instruction fetch in kernel mode
> >     #PF: error_code(0x0010) - not-present page
> 
> Somebody jumped through a NULL pointer.
> 
> >     RAX: 0000000000000000 RBX: ffff906d0cc3bb40 RCX: 0000000000000abc
> >     RDX: 0000000000000089 RSI: ffff906d74623cc0 RDI: ffff906d74475df0
> >     RBP: ffff906d74475df0 R08: ffffd70b7fb24c20 R09: ffff906d066a5000
> >     R10: 0000000000000000 R11: 8080807fffffffff R12: ffff906d74623cc0
> >     R13: 0000000000000089 R14: ffffb70b82963dc0 R15: 0000000000000080
> >     FS:  00007fbc2a8f0540(0000) GS:ffff906dcf500000(0000) knlGS:0000000000000000
> >     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >     CR2: ffffffffffffffd6 CR3: 00000003c68f8001 CR4: 00000000003606e0
> >     Call Trace:
> >      __lookup_slow+0x94/0x160
> 
> And "__lookup_slow()" has two indirect calls (they aren't obvious with
> retpoline, but look for something  like
> 
>         call __x86_indirect_thunk_rax
> 
> which is the modern sad way of doing "call *%rax"). One is for
> revalidatinging an old dentry, but the one I _suspect_ you trigger is
> this one:
> 
>                 old = inode->i_op->lookup(inode, dentry, flags);
> 
> but I thought we only could get here if we know it's a directory.
> 
> How did we miss the "d_can_lookup()", which is what should check that
> yes, we can call that ->lookup() routine.

I'll try applying a trivial patch to add d_can_lookup() to see if it
fixes the immediate issue.

> This is why I have that suspicion that it's somehow that O_PATH fd
> opened in another process without O_PATH causes confusion...
> 
> So what I think has happened is that because of the O_PATH thing,
> we've ended up with an inode that has never been truly opened (because
> O_PATH skips that part), but then with the /proc/<pid>/fd/xyz open, we
> now have a file descriptor that _looks_ like it is valid, and we're
> treating that inode as if it can be used.

I'm not sure I agree -- as I mentioned in my other mail, re-opening
through /proc/self/fd/$n works *very* well and has for a long time (in
fact, both LXC and runc depend on this working).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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