[PATCH] drivers/gpu/vga: allocate vga_arb_write() buffer on stack

[Dmitry Vyukov]

On Fri, Oct 14, 2016 at 3:06 PM, Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
> On Fri, Oct 14, 2016 at 02:54:59PM +0200, Dmitry Vyukov wrote:
>> Size of kmalloc() in vga_arb_write() is controlled by user.
>> Too large kmalloc() size triggers WARNING message on console.
>> Allocate the buffer on stack to avoid the WARNING.
>> The string must be small (e.g "target PCI:domain:bus:dev.fn").
>>
>> Signed-off-by: Dmitry Vyukov <dvyukov at google.com>
>> Cc: Dave Airlie <airlied at gmail.com>
>> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
>> Cc: dri-devel at lists.freedesktop.org
>> Cc: syzkaller at googlegroups.com
>>
>> --
>>
>> I've sent a patch that adds __GFP_NOWARN to the kmalloc a while ago:
>> https://groups.google.com/forum/#!msg/syzkaller/navdAwe4iCo/mZDhODsnAAAJ
>>
>> Ville suggested to allocate the buffer on stack as it must be small:
>>
>> "From the looks of things the longest command could be the
>> "target PCI:domain:bus:dev.fn" thing. Even assuming something silly like
>> having 10 characters for each domain,bus,dev,fn that would still be only
>> 55 bytes. So based on that even something like 64 bytes should be more
>> than enough AFAICS. "
>
>>
>> Example WARNING:
>>
>> WARNING: CPU: 2 PID: 29322 at mm/page_alloc.c:2999
>> __alloc_pages_nodemask+0x7d2/0x1760()
>> Modules linked in:
>> CPU: 2 PID: 29322 Comm: syz-executor Tainted: G    B  4.5.0-rc1+ #283
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>>  00000000ffffffff ffff880069eff670 ffffffff8299a06d 0000000000000000
>>  ffff8800658a4740 ffffffff864985a0 ffff880069eff6b0 ffffffff8134fcf9
>>  ffffffff8166de32 ffffffff864985a0 0000000000000bb7 00000000024040c0
>> Call Trace:
>>  [<     inline     >] __dump_stack lib/dump_stack.c:15
>>  [<ffffffff8299a06d>] dump_stack+0x6f/0xa2 lib/dump_stack.c:50
>>  [<ffffffff8134fcf9>] warn_slowpath_common+0xd9/0x140 kernel/panic.c:482
>>  [<ffffffff8134ff29>] warn_slowpath_null+0x29/0x30 kernel/panic.c:515
>>  [<     inline     >] __alloc_pages_slowpath mm/page_alloc.c:2999
>>  [<ffffffff8166de32>] __alloc_pages_nodemask+0x7d2/0x1760 mm/page_alloc.c:3253
>>  [<ffffffff81745c99>] alloc_pages_current+0xe9/0x450 mm/mempolicy.c:2090
>>  [<     inline     >] alloc_pages include/linux/gfp.h:459
>>  [<ffffffff81669bb6>] alloc_kmem_pages+0x16/0x100 mm/page_alloc.c:3433
>>  [<ffffffff816c20af>] kmalloc_order+0x1f/0x80 mm/slab_common.c:1008
>>  [<ffffffff816c212f>] kmalloc_order_trace+0x1f/0x140 mm/slab_common.c:1019
>>  [<     inline     >] kmalloc_large include/linux/slab.h:395
>>  [<ffffffff81756b24>] __kmalloc+0x2f4/0x340 mm/slub.c:3557
>>  [<     inline     >] kmalloc include/linux/slab.h:468
>>  [<ffffffff832c65a4>] vga_arb_write+0xd4/0xe40 drivers/gpu/vga/vgaarb.c:926
>>  [<ffffffff817a9831>] do_loop_readv_writev+0x141/0x1e0 fs/read_write.c:719
>>  [<ffffffff817ad698>] do_readv_writev+0x5f8/0x6e0 fs/read_write.c:849
>>  [<ffffffff817ad8b6>] vfs_writev+0x86/0xc0 fs/read_write.c:886
>>  [<     inline     >] SYSC_writev fs/read_write.c:919
>>  [<ffffffff817b0a21>] SyS_writev+0x111/0x2b0 fs/read_write.c:911
>>  [<ffffffff86359636>] entry_SYSCALL_64_fastpath+0x16/0x7a
>> arch/x86/entry/entry_64.S:185
>> ---
>>  drivers/gpu/vga/vgaarb.c | 11 +++--------
>>  1 file changed, 3 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/gpu/vga/vgaarb.c b/drivers/gpu/vga/vgaarb.c
>> index 1887f19..0c73083 100644
>> --- a/drivers/gpu/vga/vgaarb.c
>> +++ b/drivers/gpu/vga/vgaarb.c
>> @@ -1022,17 +1022,14 @@ static ssize_t vga_arb_write(struct file *file, const char __user *buf,
>>
>>       unsigned int io_state;
>>
>> -     char *kbuf, *curr_pos;
>> +     char kbuf[64], *curr_pos;
>>       size_t remaining = count;
>>
>>       int ret_val;
>>       int i;
>>
>> -
>> -     kbuf = kmalloc(count + 1, GFP_KERNEL);
>> -     if (!kbuf)
>> -             return -ENOMEM;
>> -
>> +     if (count >= sizeof(kbuf))
>> +             return -EINVAL;
>>       if (copy_from_user(kbuf, buf, count)) {
>>               kfree(kbuf);
>
> You missed one kfree() here.

Ouch!

> Assuming my analysis of the max string length wasn't full of crap
> I think this looks fine otherwise.
>
> So with the lingering kfree() nuked this is
> Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Mailed v2.
Thanks!


>>               return -EFAULT;
>> @@ -1259,11 +1256,9 @@ static ssize_t vga_arb_write(struct file *file, const char __user *buf,
>>               goto done;
>>       }
>>       /* If we got here, the message written is not part of the protocol! */
>> -     kfree(kbuf);
>>       return -EPROTO;
>>
>>  done:
>> -     kfree(kbuf);
>>       return ret_val;
>>  }
>>
>> --
>> 2.8.0.rc3.226.g39d4020
>
> --
> Ville Syrjälä
> Intel OTC