netdev - [PATCH bpf 2/2] libbpf: fix possible use after free in xsk_socket_

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Message-Id: <1604396490-12129-3-git-send-email-magnus.karlsson@gmail.com>
Date:   Tue,  3 Nov 2020 10:41:30 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     magnus.karlsson@...el.com, bjorn.topel@...el.com, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org,
        jonathan.lemon@...il.com, andrii.nakryiko@...il.com
Cc:     bpf@...r.kernel.org
Subject: [PATCH bpf 2/2] libbpf: fix possible use after free in xsk_socket__delete

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

Fix a possible use after free in xsk_socket__delete that will happen
if xsk_put_ctx() frees the ctx. To fix, save the umem reference taken
from the context and just use that instead.

Fixes: 2f6324a3937f ("libbpf: Support shared umems between queues and devices")
Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
---
 tools/lib/bpf/xsk.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
index 504b7a8..9bc537d 100644
--- a/tools/lib/bpf/xsk.c
+++ b/tools/lib/bpf/xsk.c
@@ -892,6 +892,7 @@ void xsk_socket__delete(struct xsk_socket *xsk)
 {
 	size_t desc_sz = sizeof(struct xdp_desc);
 	struct xdp_mmap_offsets off;
+	struct xsk_umem *umem;
 	struct xsk_ctx *ctx;
 	int err;
 
@@ -899,6 +900,7 @@ void xsk_socket__delete(struct xsk_socket *xsk)
 		return;
 
 	ctx = xsk->ctx;
+	umem = ctx->umem;
 	if (ctx->prog_fd != -1) {
 		xsk_delete_bpf_maps(xsk);
 		close(ctx->prog_fd);
@@ -918,11 +920,11 @@ void xsk_socket__delete(struct xsk_socket *xsk)
 
 	xsk_put_ctx(ctx);
 
-	ctx->umem->refcount--;
+	umem->refcount--;
 	/* Do not close an fd that also has an associated umem connected
 	 * to it.
 	 */
-	if (xsk->fd != ctx->umem->fd)
+	if (xsk->fd != umem->fd)
 		close(xsk->fd);
 	free(xsk);
 }
-- 
2.7.4