linux-kernel - Re: [PATCH] tpm: Fix NULL pointer dereference in tpm

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Date:   Thu, 5 Jul 2018 18:21:43 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     "S, Shirish" <sshankar@....com>
Cc:     Shirish S <shirish.s@....com>, linux-integrity@...r.kernel.org,
        Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: Fix NULL pointer dereference in tpm_transmit()

On Thu, Jul 05, 2018 at 09:29:34AM +0530, S, Shirish wrote:
> 
> 
> On 7/4/2018 10:43 PM, Jarkko Sakkinen wrote:
> > On Wed, Jul 04, 2018 at 02:33:40PM +0530, Shirish S wrote:
> > > During system shutdown,
> > > tpm_class_shutdown() when called with TPM_CHIP_FLAG_TPM2
> > > flag set, makes chip->ops NULL.
> > > 
> > > However tpm_chip_unregister() called later in shutdown
> > > sequence tries to access chip->ops in tpm_try_transmit()
> > > leading the NULL pointer dereference.
> > > 
> > > This patch fixes this issue.
> > > Below is the trace for reference:
> > > 
> > > BUG: unable to handle kernel NULL pointer dereference at
> > > 0000000000000048
> > > IP: tpm_transmit+0x267/0x565
> > > PGD 0 P4D 0
> > > Oops: 0000 [#1] PREEMPT SMP NOPTI
> > > ...
> > > task: ffff937c847fe580 task.stack: ffffa79f80b04000
> > > RIP: 0010:tpm_transmit+0x267/0x565
> > > RSP: 0018:ffffa79f80b07c08 EFLAGS: 00010286
> > > RAX: 0000000000000000 RBX: ffff937ca9bc8000 RCX: ffff937c847fe580
> > > RDX: 0000000000000000 RSI: 0000000000000002 RDI: ffffffff98e3cd40
> > > RBP: ffffa79f80b07c88 R08: 000000000001fff4 R09: 0000000000000000
> > > R10: 0000000000000000 R11: 0000000000000000 R12: ffffa79f80b07cd4
> > > R13: 000000000000008c R14: ffffffffffffffc3 R15: 0000000000000000
> > > FS:  00007ef31f747740(0000) GS:ffff937caed00000(0000)
> > > knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 0000000000000048 CR3: 00000001243d2000 CR4: 00000000001406e0
> > > Call Trace:
> > > tpm_transmit_cmd+0x25/0x70
> > > tpm2_shutdown+0x69/0xa3
> > > ? __radix_tree_replace+0xd9/0x120
> > > ? idr_replace_ext+0x92/0xb6
> > > tpm_chip_unregister+0xaa/0xdb
> > > cr50_i2c_shutdown+0x1e/0x41
> > > device_shutdown+0x157/0x193
> > > kernel_power_off+0x35/0x6e
> > > SYSC_reboot+0x120/0x1a3
> > > ? do_writepages+0x36/0x6e
> > > ? do_writepages+0x36/0x6e
> > > ? sync_inodes_one_sb+0x17/0x17
> > > ? _raw_spin_unlock+0xe/0x20
> > > ? iput+0x87/0x1bd
> > > do_syscall_64+0x64/0x72
> > > entry_SYSCALL_64_after_hwframe+0x3d/0xa2
> > > 
> > > Signed-off-by: Shirish S <shirish.s@....com>
> > > ---
> > >   drivers/char/tpm/tpm-interface.c | 8 ++++++++
> > >   1 file changed, 8 insertions(+)
> > > 
> > > diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> > > index e32f6e8..b14d196 100644
> > > --- a/drivers/char/tpm/tpm-interface.c
> > > +++ b/drivers/char/tpm/tpm-interface.c
> > > @@ -411,6 +411,14 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip,
> > >   	unsigned long stop;
> > >   	bool need_locality;
> > > +	/* chip->ops is made NULL in tpm_class_shutdown()
> > > +	 * This case is hit when tpm_chip_unregister() is called post
> > > +	 * tpm_class_shutdown(), hence exit early and return
> > > +	 * transmit operation not permitted
> > > +	 */
> > > +	if (!chip->ops)
> > > +		return -EPERM;
> > > +
> > >   	rc = tpm_validate_command(chip, space, buf, bufsiz);
> > >   	if (rc == -EINVAL)
> > >   		return rc;
> > > -- 
> > > 2.7.4
> > > 
> > What is cr50 anyway?
> Please ignore this patch, i realised the cr50 is not yet upstreamed.
> Hence the issue lies in in cr50 and not tpm driver.
> Thanks.
> Regards,
> Shirish S

tpm_chip_unregister() in supposed to be called only in removal/detach
sequence so the root cause for the NULL pointer deref is the driver, not
the existing TPM stack.

/Jarkko