[PATCH v2] drivers/gpu/vga: allocate vga_arb_write() buffer on stack

[Dmitry Vyukov]

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>
Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.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

---

Changes since v1:
 - removed another kfree(kbuf)

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 | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/vga/vgaarb.c b/drivers/gpu/vga/vgaarb.c
index 1887f19..77657a8 100644
--- a/drivers/gpu/vga/vgaarb.c
+++ b/drivers/gpu/vga/vgaarb.c
@@ -1022,21 +1022,16 @@ 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 (copy_from_user(kbuf, buf, count)) {
-		kfree(kbuf);
+	if (count >= sizeof(kbuf))
+		return -EINVAL;
+	if (copy_from_user(kbuf, buf, count))
 		return -EFAULT;
-	}
 	curr_pos = kbuf;
 	kbuf[count] = '\0';	/* Just to make sure... */
 
@@ -1259,11 +1254,9 @@ 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