From 37e3b24d90278288b6b6216da249b5d45fc4c411 Mon Sep 17 00:00:00 2001 From: David Gibson Date: Fri, 26 Jul 2024 17:20:30 +1000 Subject: tap: Correctly handle frames of odd length The Qemu socket protocol consists of a 32-bit frame length in network (BE) order, followed by the Ethernet frame itself. As far as I can tell, frames can be any length, with no particular alignment requirement. This means that although pkt_buf itself is aligned, if we have a frame of odd length, frames after it will have their frame length at an unaligned address. Currently we load the frame length by just casting a char pointer to (uint32_t *) and loading. Some platforms will generate a fatal trap on such an unaligned load. Even if they don't casting an incorrectly aligned pointer to (uint32_t *) is undefined behaviour, strictly speaking. Introduce a new helper to safely load a possibly unaligned value here. We assume that the compiler is smart enough to optimize this into nothing on platforms that provide performant unaligned loads. If that turns out not to be the case, we can look at improvements then. Signed-off-by: David Gibson Signed-off-by: Stefano Brivio --- tap.c | 2 +- util.h | 16 ++++++++++++++++ 2 files changed, 17 insertions(+), 1 deletion(-) diff --git a/tap.c b/tap.c index a47f9d3..7d9113a 100644 --- a/tap.c +++ b/tap.c @@ -1011,7 +1011,7 @@ void tap_handler_passt(struct ctx *c, uint32_t events, } while (n > (ssize_t)sizeof(uint32_t)) { - uint32_t l2len = ntohl(*(uint32_t *)p); + uint32_t l2len = ntohl_unaligned(p); if (l2len < sizeof(struct ethhdr) || l2len > ETH_MAX_MTU) { err("Bad frame size from guest, resetting connection"); diff --git a/util.h b/util.h index 6f44eab..b7541ce 100644 --- a/util.h +++ b/util.h @@ -10,8 +10,10 @@ #include #include #include +#include #include #include +#include #include "log.h" @@ -116,6 +118,20 @@ #define htonl_constant(x) (__bswap_constant_32(x)) #endif +/** + * ntohl_unaligned() - Read 32-bit BE value from a possibly unaligned address + * @p: Pointer to the BE value in memory + * + * Returns: Host-order value of 32-bit BE quantity at @p + */ +static inline uint32_t ntohl_unaligned(const void *p) +{ + uint32_t val; + + memcpy(&val, p, sizeof(val)); + return ntohl(val); +} + #define NS_FN_STACK_SIZE (RLIMIT_STACK_VAL * 1024 / 8) int do_clone(int (*fn)(void *), char *stack_area, size_t stack_size, int flags, void *arg); -- cgit v1.2.3