[PATCH bpf v2 1/2] bpf: Fix bpf_dynptr_copy() for overlapping ranges in one packet

[Ayushman Rout]

When both dynptrs point into the same packet and the ranges overlap,
bpf_dynptr_copy() can copy wrong data in two ways.

If one range fits into a segment and the other one does not, the slice
of the first range is used as the buffer for __bpf_dynptr_read() or
__bpf_dynptr_write(). The range that crosses into a frag is copied piece
by piece, and an earlier piece can overwrite bytes that a later piece
has yet to read.

If neither range fits into a segment, the chunks are copied from the
start to the end through a 256 byte stack buffer. When the destination
is above the source, a chunk written to the destination overwrites
source bytes that later chunks have yet to read. For a copy of 1200
bytes with the destination 100 bytes above the source, the first chunk
writes source bytes 100..355 and the second chunk then reads bytes
256..355 after they were overwritten.

Do not use a slice as the buffer when both dynptrs share the same data,
and copy the chunks from the end to the start when the destination is
above the source, like memmove() does.

The selftests added later in this series pass with this change.

Fixes: daec295a7094 ("bpf/helpers: Introduce bpf_dynptr_copy kfunc")
Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Ayushman Rout <ayushmanrout27@xxxxxxxxx>
---
 kernel/bpf/helpers.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 648abe1ba63a..b74c03669b5f 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3291,8 +3291,10 @@ __bpf_kfunc int bpf_dynptr_copy(const struct bpf_dynptr *dst_ptr, u64 dst_off,
 {
 	const struct bpf_dynptr_kern *dst = (struct bpf_dynptr_kern *)dst_ptr;
 	const struct bpf_dynptr_kern *src = (struct bpf_dynptr_kern *)src_ptr;
+	bool same = src->data == dst->data;
 	void *src_slice, *dst_slice;
 	char buf[256];
+	bool rev;
 	u64 off;
 
 	src_slice = bpf_dynptr_slice(src_ptr, src_off, NULL, size);
@@ -3303,25 +3305,31 @@ __bpf_kfunc int bpf_dynptr_copy(const struct bpf_dynptr *dst_ptr, u64 dst_off,
 		return 0;
 	}
 
-	if (src_slice)
+	/* The other range is copied piece by piece. A slice of the same packet
+	 * may overlap with it, so it cannot be the buffer for that.
+	 */
+	if (src_slice && !same)
 		return __bpf_dynptr_write(dst, dst_off, src_slice, size, 0);
 
-	if (dst_slice)
+	if (dst_slice && !same)
 		return __bpf_dynptr_read(dst_slice, size, src, src_off, 0);
 
 	if (bpf_dynptr_check_off_len(dst, dst_off, size) ||
 	    bpf_dynptr_check_off_len(src, src_off, size))
 		return -E2BIG;
 
+	/* like memmove(): from the end if the destination is above the source */
+	rev = same && dst->offset + dst_off > src->offset + src_off;
 	off = 0;
 	while (off < size) {
 		u64 chunk_sz = min_t(u64, sizeof(buf), size - off);
+		u64 pos = rev ? size - off - chunk_sz : off;
 		int err;
 
-		err = __bpf_dynptr_read(buf, chunk_sz, src, src_off + off, 0);
+		err = __bpf_dynptr_read(buf, chunk_sz, src, src_off + pos, 0);
 		if (err)
 			return err;
-		err = __bpf_dynptr_write(dst, dst_off + off, buf, chunk_sz, 0);
+		err = __bpf_dynptr_write(dst, dst_off + pos, buf, chunk_sz, 0);
 		if (err)
 			return err;
 
-- 
2.54.0