linux-kernel - [PATCH] bcache: prevent potential division by zero error

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Message-Id: <20230906012249.49203-1-rand.sec96@gmail.com>
Date:   Wed,  6 Sep 2023 04:22:49 +0300
From:   Rand Deeb <rand.sec96@...il.com>
To:     Coly Li <colyli@...e.de>,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     deeb.rand@...fident.ru, lvc-project@...uxtesting.org,
        voskresenski.stanislav@...fident.ru,
        Rand Deeb <rand.sec96@...il.com>
Subject: [PATCH] bcache: prevent potential division by zero error

In SHOW(), the variable 'n' is of type 'size_t.' While there is a
conditional check to verify that 'n' is not equal to zero before
executing the 'do_div' macro, concerns arise regarding potential
division by zero error in 64-bit environments.

The concern arises when 'n' is 64 bits in size, greater than zero, and
the lower 32 bits of it are zeros. In such cases, the conditional check
passes because 'n' is non-zero, but the 'do_div' macro casts 'n' to
'uint32_t,' effectively truncating it to its lower 32 bits.
Consequently, the 'n' value becomes zero.

To fix this potential division by zero error and ensure precise
division handling, this commit replaces the 'do_div' macro with
div64_u64(). div64_u64() is designed to work with 64-bit operands,
guaranteeing that division is performed correctly.

This change enhances the robustness of the code, ensuring that division
operations yield accurate results in all scenarios, eliminating the
possibility of division by zero, and improving compatibility across
different 64-bit environments.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Rand Deeb <rand.sec96@...il.com>
---
 drivers/md/bcache/sysfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index 554e3afc9b68..ca3e2f000cd4 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -1078,7 +1078,7 @@ SHOW(__bch_cache)
 			sum += INITIAL_PRIO - cached[i];

 		if (n)
-			do_div(sum, n);
+			sum = div64_u64(sum, n);

 		for (i = 0; i < ARRAY_SIZE(q); i++)
 			q[i] = INITIAL_PRIO - cached[n * (i + 1) /
-- 
2.34.1