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Date:   Fri, 22 Oct 2021 10:09:45 +0300
From:   Mike Rapoport <rppt@kernel.org>
To:     Kees Cook <keescook@chromium.org>,
        Jordy Zomer <jordy@jordyzomer.github.io>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        Dmitry Vyukov <dvyukov@google.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        David Hildenbrand <david@redhat.com>,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] mm/secretmem: Avoid letting secretmem_users drop to zero
Message-ID: <YXJjuWyY0h+Qaj5U@kernel.org>
References: <20211021154046.880251-1-keescook@chromium.org>
 <20211021195311.6058b90f573641542605dae4@linux-foundation.org>
 <202110212037.E18CD404@keescook>
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In-Reply-To: <202110212037.E18CD404@keescook>
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On Thu, Oct 21, 2021 at 08:39:03PM -0700, Kees Cook wrote:
> On Thu, Oct 21, 2021 at 07:53:11PM -0700, Andrew Morton wrote:
> > On Thu, 21 Oct 2021 08:40:46 -0700 Kees Cook <keescook@chromium.org> wrote:
> > 
> > > Quoting Dmitry: "refcount_inc() needs to be done before fd_install().
> > > After fd_install() finishes, the fd can be used by userspace and we can
> > > have secret data in memory before the refcount_inc().
> > > 
> > > A straightforward mis-use where a user will predict the returned fd
> > > in another thread before the syscall returns and will use it to store
> > > secret data is somewhat dubious because such a user just shoots themself
> > > in the foot.
> > > 
> > > But a more interesting mis-use would be to close the predicted fd and
> > > decrement the refcount before the corresponding refcount_inc, this way
> > > one can briefly drop the refcount to zero while there are other users
> > > of secretmem."
> > > 
> > > Move fd_install() after refcount_inc().
> > 
> > I added cc:stable.  Or doesn't the benefit/risk ratio justify that?
> 
> I hadn't because commit 110860541f44 ("mm/secretmem: use refcount_t
> instead of atomic_t") wasn't, and this would build on top of it.

Hmm, the commit 110860541f44 ("mm/secretmem: use refcount_t instead of
atomic_t") causes the splats below. I wonder if it was tested at all :(

[   20.957833] ------------[ cut here ]------------
[   20.957844] refcount_t: addition on 0; use-after-free.
[   20.957897] WARNING: CPU: 3 PID: 598 at /home/rppt/git/linux/lib/refcount.c:25 refcount_warn_saturate+0xcf/0xf0
[   20.957919] Modules linked in:
[   20.957930] CPU: 3 PID: 598 Comm: secretmemfd Not tainted 5.15.0-rc6+ #432
[   20.957944] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[   20.957948] RIP: 0010:refcount_warn_saturate+0xcf/0xf0
[   20.957957] Code: 01 01 e8 d4 db c3 ff 0f 0b c3 80 3d 39 32 43 01 00 0f 85 6b ff ff ff 48 c7 c7 00 bc c5 af c6 05 25 32 43 01 01 e8 b1 db c3 ff <0f> 0b c3 48 c7 c7 b0 bb c5 af c6 05 10 32 43 01 01 e8 9b db c3 ff
[   20.957962] RSP: 0018:ffffb188c0583f20 EFLAGS: 00010282
[   20.957967] RAX: 0000000000000000 RBX: 0000000000000003 RCX: 0000000000000027
[   20.957971] RDX: 0000000000000000 RSI: ffff8bfefbb975b0 RDI: ffff8bfefbb975b8
[   20.957974] RBP: ffffb188c0583f48 R08: 0000000000000000 R09: 0000000000000001
[   20.957977] R10: 0000000000000003 R11: ffffb188c0583d38 R12: 0000000000000000
[   20.957980] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[   20.957983] FS:  00007f9467b9c740(0000) GS:ffff8bfefbb80000(0000) knlGS:0000000000000000
[   20.957993] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   20.957997] CR2: 00007ffe83be8084 CR3: 00000001100cc003 CR4: 0000000000060ee0
[   20.958001] Call Trace:
[   20.959285]  __x64_sys_memfd_secret+0xa9/0xc0
[   20.959308]  do_syscall_64+0x3a/0x80
[   20.959331]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[   20.959352] RIP: 0033:0x7f9467cba89d
[   20.959358] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d c3 f5 0c 00 f7 d8 64 89 01 48
[   20.959362] RSP: 002b:00007ffe83bb8148 EFLAGS: 00000206 ORIG_RAX: 00000000000001bf
[   20.959368] RAX: ffffffffffffffda RBX: 0000561f62400d50 RCX: 00007f9467cba89d
[   20.959372] RDX: 0000000000000e11 RSI: 0000000000008000 RDI: 0000000000000000
[   20.959375] RBP: 00007ffe83bb8160 R08: 000000002c06910a R09: 0000000000000000
[   20.959378] R10: 00007f9467d8a1c4 R11: 0000000000000206 R12: 0000561f624008d0
[   20.959381] R13: 00007ffe83bb82b0 R14: 0000000000000000 R15: 0000000000000000
[   20.959386] ---[ end trace 9368244c7159e4de ]---
[   20.960666] ------------[ cut here ]------------
[   20.960675] refcount_t: decrement hit 0; leaking memory.
[   20.960717] WARNING: CPU: 1 PID: 598 at /home/rppt/git/linux/lib/refcount.c:31 refcount_warn_saturate+0x4f/0xf0
[   20.960737] Modules linked in:
[   20.960742] CPU: 1 PID: 598 Comm: secretmemfd Tainted: G        W         5.15.0-rc6+ #432
[   20.960748] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[   20.960751] RIP: 0010:refcount_warn_saturate+0x4f/0xf0
[   20.960759] Code: 00 00 f3 c3 83 fe 03 74 43 83 fe 04 75 1f 80 3d b3 32 43 01 00 75 eb 48 c7 c7 58 bc c5 af c6 05 a3 32 43 01 01 e8 31 dc c3 ff <0f> 0b c3 80 3d 93 32 43 01 00 75 cc 48 c7 c7 88 bc c5 af c6 05 83
[   20.960764] RSP: 0018:ffffb188c0583e40 EFLAGS: 00010286
[   20.960769] RAX: 0000000000000000 RBX: ffff8bfec1f51900 RCX: 0000000000000027
[   20.960772] RDX: 0000000000000000 RSI: ffff8bfefba975b0 RDI: ffff8bfefba975b8
[   20.960775] RBP: 0000000000080003 R08: 0000000000000000 R09: 0000000000000001
[   20.960778] R10: ffff8bfec439da80 R11: ffffb188c0583c58 R12: ffff8bfec4e576a0
[   20.960781] R13: ffff8bfec01a8ca0 R14: ffff8bfecd314300 R15: 0000000000000000
[   20.960784] FS:  0000000000000000(0000) GS:ffff8bfefba80000(0000) knlGS:0000000000000000
[   20.960835] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   20.960840] CR2: 00007f9467c85290 CR3: 0000000080a0c004 CR4: 0000000000060ee0
[   20.960843] Call Trace:
[   20.960849]  secretmem_release+0x26/0x30
[   20.960862]  __fput+0x85/0x240
[   20.960868]  task_work_run+0x67/0xa0
[   20.960890]  do_exit+0x363/0xbb0
[   20.960902]  do_group_exit+0x35/0x90
[   20.960908]  __x64_sys_exit_group+0xf/0x10
[   20.960913]  do_syscall_64+0x3a/0x80
[   20.960922]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[   20.960928] RIP: 0033:0x7f9467c852c6
[   20.960933] Code: Unable to access opcode bytes at RIP 0x7f9467c8529c.
[   20.960936] RSP: 002b:00007ffe83bb8168 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
[   20.960941] RAX: ffffffffffffffda RBX: 00007f9467d8c610 RCX: 00007f9467c852c6
[   20.960944] RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
[   20.960947] RBP: 0000000000000000 R08: 00000000000000e7 R09: ffffffffffffff80
[   20.960950] R10: 0000000000000003 R11: 0000000000000246 R12: 00007f9467d8c610
[   20.960953] R13: 0000000000000001 R14: 00007f9467d8ffc8 R15: 0000000000000000
[   20.960957] ---[ end trace 9368244c7159e4df ]---
 
> I think the exposure is very small in both places, so probably best to
> avoid the churn, but I'm not _opposed_ to it.
> 
> -- 
> Kees Cook

-- 
Sincerely yours,
Mike.