[PATCH] accel/ethosu: fix OOB write in ethosu_gem_cmdstream_copy_and_validate()

[Muhammad Bilal <meatuni001@gmail.com>]

The command stream parsing loop increments the index variable a second
time when a 64-bit command word is encountered (bit 14 set), but does
not re-check the loop bound before writing the second word:

    for (i = 0; i < size / 4; i++) {
        bocmds[i] = cmds[0];
        if (cmd & 0x4000) {
            i++;
            bocmds[i] = cmds[1];   /* unchecked */
        }
    }

The buffer bocmds is backed by a DMA allocation of exactly size bytes
from drm_gem_dma_create(ddev, size), giving valid indices [0, size/4-1].

When i == size/4 - 1 on entry to an iteration and bit 14 of cmds[0] is
set, bocmds[size/4-1] is written in bounds, i is then incremented to
size/4, and bocmds[size/4] writes four bytes past the end of the
allocation.

Userspace controls both the buffer contents and the size argument via
the ioctl, making this a userspace-triggerable heap out-of-bounds write.

Fix by checking the incremented index against the buffer bound before
the second write and returning -EINVAL if the buffer is too small to
contain the extended command.

Fixes: 5a5e9c0228e6 ("accel: Add Arm Ethos-U NPU driver")
Cc: stable@vger.kernel.org
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
---
 drivers/accel/ethosu/ethosu_gem.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/accel/ethosu/ethosu_gem.c b/drivers/accel/ethosu/ethosu_gem.c
index 7994e7073903..f526f4aedffd 100644
--- a/drivers/accel/ethosu/ethosu_gem.c
+++ b/drivers/accel/ethosu/ethosu_gem.c
@@ -387,6 +387,8 @@ static int ethosu_gem_cmdstream_copy_and_validate(struct drm_device *ddev,
 				return -EFAULT;
 
 			i++;
+			if (i >= size / 4)
+				return -EINVAL;
 			bocmds[i] = cmds[1];
 			addr = cmd_to_addr(cmds);
 		}
-- 
2.53.0