[Qemu-devel] [PATCH] linux-user: Handle short lengths in host_to_target_sockaddr()

[Peter Maydell <peter.maydell@linaro.org>]

If userspace specifies a short buffer for a target sockaddr,
the kernel will only copy in as much as it has space for
(or none at all if the length is zero) -- see the kernel
move_addr_to_user() function. Mimic this in QEMU's
host_to_target_sockaddr() routine.

In particular, this fixes a segfault running the LTP
recvfrom01 test, where the guest makes a recvfrom()
call with a bad buffer pointer and other parameters which
cause the kernel to set the addrlen to zero; because we
did not skip the attempt to swap the sa_family field we
segfaulted on the bad address.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 linux-user/syscall.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 165fd06..6e77d34 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -1374,12 +1374,19 @@ static inline abi_long host_to_target_sockaddr(abi_ulong target_addr,
 {
     struct target_sockaddr *target_saddr;
 
+    if (len == 0) {
+        return 0;
+    }
+
     target_saddr = lock_user(VERIFY_WRITE, target_addr, len, 0);
     if (!target_saddr)
         return -TARGET_EFAULT;
     memcpy(target_saddr, addr, len);
-    target_saddr->sa_family = tswap16(addr->sa_family);
-    if (addr->sa_family == AF_NETLINK) {
+    if (len >= offsetof(struct target_sockaddr, sa_family) +
+        sizeof(target_saddr->sa_family)) {
+        target_saddr->sa_family = tswap16(addr->sa_family);
+    }
+    if (addr->sa_family == AF_NETLINK && len >= sizeof(struct sockaddr_nl)) {
         struct sockaddr_nl *target_nl = (struct sockaddr_nl *)target_saddr;
         target_nl->nl_pid = tswap32(target_nl->nl_pid);
         target_nl->nl_groups = tswap32(target_nl->nl_groups);
-- 
1.9.1