[PATCH next] dma-buf/sync-file: do not allow zero size allocations

[Pavel Skripkin <paskripkin@gmail.com>]

syzbot reported GPF in dma_fence_array_first(), which is caused by
dereferencing ZERO_PTR in dma-buf internals.

ZERO_PTR was generated in sync_file_merge(). This functuion tries to
reduce allocation size, but does not check if it reducing to 0.

Fix reported bug by validating `index` value before passing it to
krealloc_array().

Fail log:

general protection fault, probably for non-canonical address 0xdffffc0000000002: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
CPU: 1 PID: 3595 Comm: syz-executor814 Not tainted 5.17.0-next-20220328-syzkaller #0
...
RIP: 0010:dma_fence_array_first+0x78/0xb0 drivers/dma-buf/dma-fence-array.c:234
...
Call Trace:
 <TASK>
 __dma_fence_unwrap_array include/linux/dma-fence-unwrap.h:42 [inline]
 dma_fence_unwrap_first include/linux/dma-fence-unwrap.h:57 [inline]
 sync_file_ioctl_fence_info drivers/dma-buf/sync_file.c:414 [inline]
 sync_file_ioctl+0x248/0x22c0 drivers/dma-buf/sync_file.c:477
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:870 [inline]

There was same problem with initial kcalloc() allocation in same
function, so it's fixed as well.

Reported-and-tested-by: syzbot+5c943fe38e86d615cac2@syzkaller.appspotmail.com
Fixes: 519f490db07e ("dma-buf/sync-file: fix warning about fence containers")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
---
 drivers/dma-buf/sync_file.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
index b8dea4ec123b..aa744f017008 100644
--- a/drivers/dma-buf/sync_file.c
+++ b/drivers/dma-buf/sync_file.c
@@ -212,7 +212,7 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
 	dma_fence_unwrap_for_each(b_fence, &b_iter, b->fence)
 		++num_fences;
 
-	if (num_fences > INT_MAX)
+	if (num_fences > INT_MAX || !num_fences)
 		goto err_free_sync_file;
 
 	fences = kcalloc(num_fences, sizeof(*fences), GFP_KERNEL);
@@ -264,7 +264,7 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
 	if (index == 0)
 		add_fence(fences, &index, dma_fence_get_stub());
 
-	if (num_fences > index) {
+	if (index && num_fences > index) {
 		struct dma_fence **tmp;
 
 		/* Keep going even when reducing the size failed */
-- 
2.35.1