Re: [PATCH] crypto: blkcipher_get_spot() handling of buffer at end of page

From: Herbert Xu
Date: Sat Sep 08 2007 - 00:14:44 EST


On Fri, Sep 07, 2007 at 05:09:17PM -0700, Bob Gilligan wrote:
>
> Proposed patch is below. We found the problem and tested this fix in
> 2.6.20, but it looks like the relevant code in blkcipher.c is the same
> in the latest tree.

Good catch!

I've fixed this slightly differently. Also, the kmalloc size
in the case where it does straddle a page isn't enough either.

[CRYPTO] blkcipher: Fix handling of kmalloc page straddling

The function blkcipher_get_spot tries to return a buffer of
the specified length that does not straddle a page. It has
an off-by-one bug so it may advance a page unnecessarily.

What's worse, one of its callers doesn't provide a buffer
that's sufficiently long for this operation.

This patch fixes both problems. Thanks to Bob Gilligan for
diagnosing this problem and providing a fix.

Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

Cheers,
--
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Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
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--
diff --git a/crypto/blkcipher.c b/crypto/blkcipher.c
index 7755834..469cb7f 100644
--- a/crypto/blkcipher.c
+++ b/crypto/blkcipher.c
@@ -59,11 +59,13 @@ static inline void blkcipher_unmap_dst(struct blkcipher_walk *walk)
scatterwalk_unmap(walk->dst.virt.addr, 1);
}

+/* Get a spot of the specified length that does not straddle a page.
+ * The caller needs to ensure that there is enough space for this operation.
+ */
static inline u8 *blkcipher_get_spot(u8 *start, unsigned int len)
{
- if (offset_in_page(start + len) < len)
- return (u8 *)((unsigned long)(start + len) & PAGE_MASK);
- return start;
+ u8 *end_page = (u8 *)(((unsigned long)(start + len - 1)) & PAGE_MASK);
+ return start < end_page ? start : end_page;
}

static inline unsigned int blkcipher_done_slow(struct crypto_blkcipher *tfm,
@@ -155,7 +157,8 @@ static inline int blkcipher_next_slow(struct blkcipher_desc *desc,
if (walk->buffer)
goto ok;

- n = bsize * 2 + (alignmask & ~(crypto_tfm_ctx_alignment() - 1));
+ n = bsize * 3 - (alignmask + 1) +
+ (alignmask & ~(crypto_tfm_ctx_alignment() - 1));
walk->buffer = kmalloc(n, GFP_ATOMIC);
if (!walk->buffer)
return blkcipher_walk_done(desc, walk, -ENOMEM);
-
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