Re: [PATCH v2] tee: add overflow check in register_shm_helper()

[Jens Wiklander]

Hi Sumit,

On Thu, Aug 18, 2022 at 2:41 PM Sumit Garg <sumit.garg@xxxxxxxxxx> wrote:
>
> Hi Jens,
>
> On Thu, 18 Aug 2022 at 16:39, Jens Wiklander <jens.wiklander@xxxxxxxxxx> wrote:
> >
> > With special lengths supplied by user space, register_shm_helper() has
> > an integer overflow when calculating the number of pages covered by a
> > supplied user space memory region. This causes
> > internal_get_user_pages_fast() a helper function of
> > pin_user_pages_fast() to do a NULL pointer dereference.
> >
> > [   14.141620] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
> > [   14.142556] Mem abort info:
> > [   14.142829]   ESR = 0x0000000096000044
> > [   14.143237]   EC = 0x25: DABT (current EL), IL = 32 bits
> > [   14.143742]   SET = 0, FnV = 0
> > [   14.144052]   EA = 0, S1PTW = 0
> > [   14.144348]   FSC = 0x04: level 0 translation fault
> > [   14.144767] Data abort info:
> > [   14.145053]   ISV = 0, ISS = 0x00000044
> > [   14.145394]   CM = 0, WnR = 1
> > [   14.145766] user pgtable: 4k pages, 48-bit VAs, pgdp=000000004278e000
> > [   14.146279] [0000000000000010] pgd=0000000000000000, p4d=0000000000000000
> > [   14.147435] Internal error: Oops: 96000044 [#1] PREEMPT SMP
> > [   14.148026] Modules linked in:
> > [   14.148595] CPU: 1 PID: 173 Comm: optee_example_a Not tainted 5.19.0 #11
> > [   14.149204] Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
> > [   14.149832] pstate: 604000c5 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > [   14.150481] pc : internal_get_user_pages_fast+0x474/0xa80
> > [   14.151640] lr : internal_get_user_pages_fast+0x404/0xa80
> > [   14.152408] sp : ffff80000a88bb30
> > [   14.152711] x29: ffff80000a88bb30 x28: 0000fffff836d000 x27: 0000fffff836e000
> > [   14.153580] x26: fffffc0000000000 x25: fffffc0000f4a1c0 x24: ffff00000289fb70
> > [   14.154634] x23: ffff000002702e08 x22: 0000000000040001 x21: ffff8000097eec60
> > [   14.155378] x20: 0000000000f4a1c0 x19: 00e800007d287f43 x18: 0000000000000000
> > [   14.156215] x17: 0000000000000000 x16: 0000000000000000 x15: 0000fffff836cfb0
> > [   14.157068] x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
> > [   14.157747] x11: 0000000000000000 x10: 0000000000000000 x9 : 0000000000000000
> > [   14.158576] x8 : ffff00000276ec80 x7 : 0000000000000000 x6 : 000000000000003f
> > [   14.159243] x5 : 0000000000000000 x4 : ffff000041ec4eac x3 : ffff000002774cb8
> > [   14.159977] x2 : 0000000000000004 x1 : 0000000000000010 x0 : 0000000000000000
> > [   14.160883] Call trace:
> > [   14.161166]  internal_get_user_pages_fast+0x474/0xa80
> > [   14.161763]  pin_user_pages_fast+0x24/0x4c
> > [   14.162227]  register_shm_helper+0x194/0x330
> > [   14.162734]  tee_shm_register_user_buf+0x78/0x120
> > [   14.163290]  tee_ioctl+0xd0/0x11a0
> > [   14.163739]  __arm64_sys_ioctl+0xa8/0xec
> > [   14.164227]  invoke_syscall+0x48/0x114
> > [   14.164653]  el0_svc_common.constprop.0+0x44/0xec
> > [   14.165130]  do_el0_svc+0x2c/0xc0
> > [   14.165498]  el0_svc+0x2c/0x84
> > [   14.165847]  el0t_64_sync_handler+0x1ac/0x1b0
> > [   14.166258]  el0t_64_sync+0x18c/0x190
> > [   14.166878] Code: 91002318 11000401 b900f7e1 f9403be1 (f820d839)
> > [   14.167666] ---[ end trace 0000000000000000 ]---
> >
> > Fix this by adding an overflow check when calculating the end of the
> > memory range. Also add an explicit call to access_ok() in
> > tee_shm_register_user_buf() to catch an invalid user space address
> > early.
> >
> > Fixes: 033ddf12bcf5 ("tee: add register user memory")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reported-by: Nimish Mishra <neelam.nimish@xxxxxxxxx>
> > Reported-by: Anirban Chakraborty <ch.anirban00727@xxxxxxxxx>
> > Reported-by: Debdeep Mukhopadhyay <debdeep.mukhopadhyay@xxxxxxxxx>
> > Suggested-by: Jerome Forissier <jerome.forissier@xxxxxxxxxx>
> > Signed-off-by: Jens Wiklander <jens.wiklander@xxxxxxxxxx>
> > ---
> >  drivers/tee/tee_shm.c | 23 +++++++++++++++++++----
> >  1 file changed, 19 insertions(+), 4 deletions(-)
> >
>
> I can't see the v1 and neither a changelog for v2, so my comments
> below may be duplicate.

Fair point. The original patch wasn't posted publicly, but in order to
avoid confusion with that patch I chose to publish this as V2.

>
> > diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
> > index f2b1bcefcadd..f71651021c8d 100644
> > --- a/drivers/tee/tee_shm.c
> > +++ b/drivers/tee/tee_shm.c
> > @@ -231,15 +231,30 @@ EXPORT_SYMBOL_GPL(tee_shm_alloc_priv_buf);
> >
> >  static struct tee_shm *
> >  register_shm_helper(struct tee_context *ctx, unsigned long addr,
> > -                   size_t length, u32 flags, int id)
> > +                   unsigned long length, u32 flags, int id)
> >  {
> >         struct tee_device *teedev = ctx->teedev;
> > +       unsigned long end_addr;
> >         struct tee_shm *shm;
> >         unsigned long start;
> >         size_t num_pages;
> >         void *ret;
> >         int rc;
> >
> > +       /* Check for overflows, this may be input from user space */
>
> IMO, this bound checking should be part of the parent function (like
> tee_shm_register_user_buf() in this case).

I don't see any harm in checking it here even if it will then check
input from tee_shm_register_kernel_buf() too. Then I'm also reusing
the result in the roundup() and that should be done in this function.

>
> > +       addr = untagged_addr(addr);
> > +       start = rounddown(addr, PAGE_SIZE);
> > +       if (check_add_overflow(addr, length, &end_addr))
> > +               return ERR_PTR(-EINVAL);
>
> Isn't this check redundant after access_ok()? AFAICS, access_ok()
> should limit the upper bound to TASK_SIZE_MAX which should detect any
> overflows.

It may be redundant, depending on the configuration. It's likely
redundant on all platforms we care about at the moment, but who knows
where this will be used in the future.

>
> > +       end_addr = roundup(end_addr, PAGE_SIZE);
> > +       if (end_addr < start)
> > +               return ERR_PTR(-EINVAL);
>
> Ditto?

Yeah, same argument.

Thanks,
Jens

>
> -Sumit
>
> > +       num_pages = (end_addr - start) / PAGE_SIZE;
> > +
> > +       /* Error out early if no pages are to be registered */
> > +       if (!num_pages)
> > +               return ERR_PTR(-EINVAL);
> > +
> >         if (!tee_device_get(teedev))
> >                 return ERR_PTR(-EINVAL);
> >
> > @@ -261,11 +276,8 @@ register_shm_helper(struct tee_context *ctx, unsigned long addr,
> >         shm->flags = flags;
> >         shm->ctx = ctx;
> >         shm->id = id;
> > -       addr = untagged_addr(addr);
> > -       start = rounddown(addr, PAGE_SIZE);
> >         shm->offset = addr - start;
> >         shm->size = length;
> > -       num_pages = (roundup(addr + length, PAGE_SIZE) - start) / PAGE_SIZE;
> >         shm->pages = kcalloc(num_pages, sizeof(*shm->pages), GFP_KERNEL);
> >         if (!shm->pages) {
> >                 ret = ERR_PTR(-ENOMEM);
> > @@ -326,6 +338,9 @@ struct tee_shm *tee_shm_register_user_buf(struct tee_context *ctx,
> >         void *ret;
> >         int id;
> >
> > +       if (!access_ok((void __user *)addr, length))
> > +               return ERR_PTR(-EFAULT);
> > +
> >         mutex_lock(&teedev->mutex);
> >         id = idr_alloc(&teedev->idr, NULL, 1, 0, GFP_KERNEL);
> >         mutex_unlock(&teedev->mutex);
> > --
> > 2.31.1
> >