linux-kernel - [PATCH] mm/usercopy: Use memory range to be accessed for wraparound check

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Message-Id: <1564509253-23287-1-git-send-email-isaacm@codeaurora.org>
Date:   Tue, 30 Jul 2019 10:54:13 -0700
From:   "Isaac J. Manjarres" <isaacm@...eaurora.org>
To:     keescook@...omium.org, crecklin@...hat.com
Cc:     "Isaac J. Manjarres" <isaacm@...eaurora.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        psodagud@...eaurora.org, tsoni@...eaurora.org,
        eberman@...eaurora.org, stable@...r.kernel.org
Subject: [PATCH] mm/usercopy: Use memory range to be accessed for wraparound check

Currently, when checking to see if accessing n bytes starting at
address "ptr" will cause a wraparound in the memory addresses,
the check in check_bogus_address() adds an extra byte, which is
incorrect, as the range of addresses that will be accessed is
[ptr, ptr + (n - 1)].

This can lead to incorrectly detecting a wraparound in the
memory address, when trying to read 4 KB from memory that is
mapped to the the last possible page in the virtual address
space, when in fact, accessing that range of memory would not
cause a wraparound to occur.

Use the memory range that will actually be accessed when
considering if accessing a certain amount of bytes will cause
the memory address to wrap around.

Fixes: f5509cc18daa ("mm: Hardened usercopy")
Co-developed-by: Prasad Sodagudi <psodagud@...eaurora.org>
Signed-off-by: Prasad Sodagudi <psodagud@...eaurora.org>
Signed-off-by: Isaac J. Manjarres <isaacm@...eaurora.org>
Cc: stable@...r.kernel.org
Reviewed-by: William Kucharski <william.kucharski@...cle.com>
Acked-by: Kees Cook <keescook@...omium.org>
---
 mm/usercopy.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/usercopy.c b/mm/usercopy.c
index 2a09796..98e92486 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -147,7 +147,7 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
 				       bool to_user)
 {
 	/* Reject if object wraps past end of memory. */
-	if (ptr + n < ptr)
+	if (ptr + (n - 1) < ptr)
 		usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);

 	/* Reject if NULL or ZERO-allocation. */
-- 
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