[PATCH bpf v2] xsk: fix broken Tx ring validation

From: Magnus Karlsson <magnus.karlsson@intel.com>

Fix broken Tx ring validation for AF_XDP. The commit under the Fixes
tag, fixed an off-by-one error in the validation but introduced
another error. Descriptors are now let through even if they straddle a
chunk boundary which they are not allowed to do in aligned mode. Worse
is that they are let through even if they straddle the end of the umem
itself, tricking the kernel to read data outside the allowed umem
region which might or might not be mapped at all.

Fix this by reintroducing the old code, but subtract the length by one
to fix the off-by-one error that the original patch was
addressing. The test chunk != chunk_end makes sure packets do not
straddle chunk boundraries. Note that packets of zero length are
allowed in the interface, therefore the test if the length is
non-zero.

v1 -> v2:
* Improved commit message

Fixes: ac31565c2193 ("xsk: Fix for xp_aligned_validate_desc() when len == chunk_size")
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
---
 net/xdp/xsk_queue.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
index 9d2a89d793c0..9ae13cccfb28 100644
--- a/net/xdp/xsk_queue.h
+++ b/net/xdp/xsk_queue.h

@@ -128,12 +128,15 @@ static inline bool xskq_cons_read_addr_unchecked(struct xsk_queue *q, u64 *addr)
 static inline bool xp_aligned_validate_desc(struct xsk_buff_pool *pool,
 					    struct xdp_desc *desc)
 {
-	u64 chunk;
-
-	if (desc->len > pool->chunk_size)
-		return false;
+	u64 chunk, chunk_end;
 
 	chunk = xp_aligned_extract_addr(pool, desc->addr);
+	if (likely(desc->len)) {
+		chunk_end = xp_aligned_extract_addr(pool, desc->addr + desc->len - 1);
+		if (chunk != chunk_end)
+			return false;
+	}
+
 	if (chunk >= pool->addrs_cnt)
 		return false;
 

base-commit: da5ac772cfe2a03058b0accfac03fad60c46c24d
-- 
2.29.0

On Fri, 18 Jun 2021 at 09:58, Magnus Karlsson [off-list ref] wrote:

From: Magnus Karlsson <magnus.karlsson@intel.com>

Fix broken Tx ring validation for AF_XDP. The commit under the Fixes
tag, fixed an off-by-one error in the validation but introduced
another error. Descriptors are now let through even if they straddle a
chunk boundary which they are not allowed to do in aligned mode. Worse
is that they are let through even if they straddle the end of the umem
itself, tricking the kernel to read data outside the allowed umem
region which might or might not be mapped at all.

Fix this by reintroducing the old code, but subtract the length by one
to fix the off-by-one error that the original patch was
addressing. The test chunk != chunk_end makes sure packets do not
straddle chunk boundraries. Note that packets of zero length are
allowed in the interface, therefore the test if the length is
non-zero.

v1 -> v2:
* Improved commit message

Fixes: ac31565c2193 ("xsk: Fix for xp_aligned_validate_desc() when len == chunk_size")
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>

Acked-by: Björn Töpel <bjorn@kernel.org>

Hello:

This patch was applied to bpf/bpf.git (refs/heads/master):

On Fri, 18 Jun 2021 09:58:05 +0200 you wrote:

From: Magnus Karlsson <magnus.karlsson@intel.com>

Fix broken Tx ring validation for AF_XDP. The commit under the Fixes
tag, fixed an off-by-one error in the validation but introduced
another error. Descriptors are now let through even if they straddle a
chunk boundary which they are not allowed to do in aligned mode. Worse
is that they are let through even if they straddle the end of the umem
itself, tricking the kernel to read data outside the allowed umem
region which might or might not be mapped at all.

[...]

Here is the summary with links:
  - [bpf,v2] xsk: fix broken Tx ring validation
    https://git.kernel.org/bpf/bpf/c/f654fae47e83

You are awesome, thank you!
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