Re: [PATCH] lib/test_hmm.c: fix harmless shift wrapping bug

From: Ralph Campbell
Date: Tue Mar 16 2021 - 12:44:40 EST

On 3/16/21 4:43 AM, Dan Carpenter wrote:
The "cmd.npages" variable is a u64 that comes from the user. I noticed
during review that it could have a shift wrapping bug when it is used
in the integer overflow test on the next line.

It turns out this is harmless. The users all do:

unsigned long size = cmd->npages << PAGE_SHIFT;

and after that "size" is used consistently and "cmd->npages" is never
used again. So even when there is an integer overflow, everything works

Even though this is harmless, I believe syzbot will complain and fixing
it makes the code easier to read.

Fixes: b2ef9f5a5cb3 ("mm/hmm/test: add selftest driver for HMM")
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
lib/test_hmm.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index 80a78877bd93..541466034a6b 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -930,6 +930,8 @@ static long dmirror_fops_unlocked_ioctl(struct file *filp,
if (cmd.addr & ~PAGE_MASK)
return -EINVAL;
+ if (cmd.npages > ULONG_MAX >> PAGE_SHIFT)
+ return -EINVAL;
if (cmd.addr >= (cmd.addr + (cmd.npages << PAGE_SHIFT)))
return -EINVAL;

Looks good to me too. Thanks for sending this.
Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>