[PATCH bpf] xsk: harden userspace-supplied &xdp_desc validation

[Alexander Lobakin <aleksander.lobakin@intel.com>]

Turned out certain clearly invalid values passed in &xdp_desc from
userspace can pass xp_{,un}aligned_validate_desc() and then lead
to UBs or just invalid frames to be queued for xmit.

desc->len close to ``U32_MAX`` with a non-zero pool->tx_metadata_len
can cause positive integer overflow and wraparound, the same way low
enough desc->addr with a non-zero pool->tx_metadata_len can cause
negative integer overflow. Both scenarios can then pass the
validation successfully.
This doesn't happen with valid XSk applications, but can be used
to perform attacks.

Always promote desc->len to ``u64`` first to exclude positive
overflows of it. Use explicit check_{add,sub}_overflow() when
validating desc->addr (which is ``u64`` already).

bloat-o-meter reports a little growth of the code size:

add/remove: 0/0 grow/shrink: 2/1 up/down: 60/-16 (44)
Function                                     old     new   delta
xskq_cons_peek_desc                          299     330     +31
xsk_tx_peek_release_desc_batch               973    1002     +29
xsk_generic_xmit                            3148    3132     -16

but hopefully this doesn't hurt the performance much.

Fixes: 341ac980eab9 ("xsk: Support tx_metadata_len")
Cc: stable@vger.kernel.org # 6.8+
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
---
 net/xdp/xsk_queue.h | 45 +++++++++++++++++++++++++++++++++++----------
 1 file changed, 35 insertions(+), 10 deletions(-)

diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
index f16f390370dc..1eb8d9f8b104 100644
--- a/net/xdp/xsk_queue.h
+++ b/net/xdp/xsk_queue.h
@@ -143,14 +143,24 @@ static inline bool xp_unused_options_set(u32 options)
 static inline bool xp_aligned_validate_desc(struct xsk_buff_pool *pool,
 					    struct xdp_desc *desc)
 {
-	u64 addr = desc->addr - pool->tx_metadata_len;
-	u64 len = desc->len + pool->tx_metadata_len;
-	u64 offset = addr & (pool->chunk_size - 1);
+	u64 len = desc->len;
+	u64 addr, offset;
 
-	if (!desc->len)
+	if (!len)
 		return false;
 
-	if (offset + len > pool->chunk_size)
+	/* Can overflow if desc->addr < pool->tx_metadata_len */
+	if (check_sub_overflow(desc->addr, pool->tx_metadata_len, &addr))
+		return false;
+
+	offset = addr & (pool->chunk_size - 1);
+
+	/*
+	 * Can't overflow: @offset is guaranteed to be < ``U32_MAX``
+	 * (pool->chunk_size is ``u32``), @len is guaranteed
+	 * to be <= ``U32_MAX``.
+	 */
+	if (offset + len + pool->tx_metadata_len > pool->chunk_size)
 		return false;
 
 	if (addr >= pool->addrs_cnt)
@@ -158,27 +168,42 @@ static inline bool xp_aligned_validate_desc(struct xsk_buff_pool *pool,
 
 	if (xp_unused_options_set(desc->options))
 		return false;
+
 	return true;
 }
 
 static inline bool xp_unaligned_validate_desc(struct xsk_buff_pool *pool,
 					      struct xdp_desc *desc)
 {
-	u64 addr = xp_unaligned_add_offset_to_addr(desc->addr) - pool->tx_metadata_len;
-	u64 len = desc->len + pool->tx_metadata_len;
+	u64 len = desc->len;
+	u64 addr, end;
 
-	if (!desc->len)
+	if (!len)
 		return false;
 
+	/* Can't overflow: @len is guaranteed to be <= ``U32_MAX`` */
+	len += pool->tx_metadata_len;
 	if (len > pool->chunk_size)
 		return false;
 
-	if (addr >= pool->addrs_cnt || addr + len > pool->addrs_cnt ||
-	    xp_desc_crosses_non_contig_pg(pool, addr, len))
+	/* Can overflow if desc->addr is close to 0 */
+	if (check_sub_overflow(xp_unaligned_add_offset_to_addr(desc->addr),
+			       pool->tx_metadata_len, &addr))
+		return false;
+
+	if (addr >= pool->addrs_cnt)
+		return false;
+
+	/* Can overflow if pool->addrs_cnt is high enough */
+	if (check_add_overflow(addr, len, &end) || end > pool->addrs_cnt)
+		return false;
+
+	if (xp_desc_crosses_non_contig_pg(pool, addr, len))
 		return false;
 
 	if (xp_unused_options_set(desc->options))
 		return false;
+
 	return true;
 }
 
-- 
2.51.0