linux-kernel - [PATCH] lib/test_hmm.c: fix harmless shift wrapping bug

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Message-ID: <YFCZ1NNfMoixOjWP@mwanda>
Date:   Tue, 16 Mar 2021 14:43:16 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Jérôme Glisse <jglisse@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: [PATCH] lib/test_hmm.c: fix harmless shift wrapping bug

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
fine.

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@...cle.com>
---
 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;

-- 
2.30.1