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

[Ville Syrjälä]

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.

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>

>  		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