On Fr, 2014-07-25 at 19:23 +0400, Andrey Ryabinin wrote: > On 07/14/14 01:50, Sasha Levin wrote: > > > > > I've tried debugging it, but I don't see a code path that could lead to > > that. > > > > I finally found some time to take look at this and I've found where the > problem is. > > Sasha, I suppose there was no usual "Unable to handle NULL pointer deference" > after KASAN's report, right? > > This gave me a clue that address 0 is actually mapped and contains valid > socket address structure in it. > I've managed to write a simple code (in attachment), which could easily > reproduce this bug. > > I've fixed it with the following patch, please take a look. > > > From: Andrey Ryabinin <a.ryabi...@samsung.com> > Subject: [PATCH] net: sendmsg: fix NULL pointer dereference > > Sasha's report: > > While fuzzing with trinity inside a KVM tools guest running the > latest -next > > kernel with the KASAN patchset, I've stumbled on the following spew: > > > > [ 4448.949424] > ================================================================== > > [ 4448.951737] AddressSanitizer: user-memory-access on address 0 > > [ 4448.952988] Read of size 2 by thread T19638: > > [ 4448.954510] CPU: 28 PID: 19638 Comm: trinity-c76 Not tainted > 3.16.0-rc4-next-20140711-sasha-00046-g07d3099-dirty #813 > > [ 4448.956823] ffff88046d86ca40 0000000000000000 ffff880082f37e78 > ffff880082f37a40 > > [ 4448.958233] ffffffffb6e47068 ffff880082f37a68 ffff880082f37a58 > ffffffffb242708d > > [ 4448.959552] 0000000000000000 ffff880082f37a88 ffffffffb24255b1 > 0000000000000000 > > [ 4448.961266] Call Trace: > > [ 4448.963158] dump_stack (lib/dump_stack.c:52) > > [ 4448.964244] kasan_report_user_access (mm/kasan/report.c:184) > > [ 4448.965507] __asan_load2 (mm/kasan/kasan.c:352) > > [ 4448.966482] ? netlink_sendmsg (net/netlink/af_netlink.c:2339) > > [ 4448.967541] netlink_sendmsg (net/netlink/af_netlink.c:2339) > > [ 4448.968537] ? get_parent_ip (kernel/sched/core.c:2555) > > [ 4448.970103] sock_sendmsg (net/socket.c:654) > > [ 4448.971584] ? might_fault (mm/memory.c:3741) > > [ 4448.972526] ? might_fault (./arch/x86/include/asm/current.h:14 > mm/memory.c:3740) > > [ 4448.973596] ? verify_iovec (net/core/iovec.c:64) > > [ 4448.974522] ___sys_sendmsg (net/socket.c:2096) > > [ 4448.975797] ? put_lock_stats.isra.13 > (./arch/x86/include/asm/preempt.h:98 kernel/locking/lockdep.c:254) > > [ 4448.977030] ? lock_release_holdtime (kernel/locking/lockdep.c:273) > > [ 4448.978197] ? lock_release_non_nested > (kernel/locking/lockdep.c:3434 (discriminator 1)) > > [ 4448.979346] ? check_chain_key (kernel/locking/lockdep.c:2188) > > [ 4448.980535] __sys_sendmmsg (net/socket.c:2181) > > [ 4448.981592] ? trace_hardirqs_on_caller > (kernel/locking/lockdep.c:2600) > > [ 4448.982773] ? trace_hardirqs_on (kernel/locking/lockdep.c:2607) > > [ 4448.984458] ? syscall_trace_enter (arch/x86/kernel/ptrace.c:1500 > (discriminator 2)) > > [ 4448.985621] ? trace_hardirqs_on_caller > (kernel/locking/lockdep.c:2600) > > [ 4448.986754] SyS_sendmmsg (net/socket.c:2201) > > [ 4448.987708] tracesys (arch/x86/kernel/entry_64.S:542) > > [ 4448.988929] > ================================================================== > > This reports means that we've come to netlink_sendmsg() with msg->msg_name == > NULL and msg->msg_namelen > 0. > > After this report there was no usual "Unable to handle kernel NULL pointer > dereference" > and this gave me a clue that address 0 is mapped and contains valid socket > address structure in it. > > This bug was introduced in f3d3342602f8bcbf37d7c46641cb9bca7618eb1c > (net: rework recvmsg handler msg_name and msg_namelen logic). > Commit message states that: > "Set msg->msg_name = NULL if user specified a NULL in msg_name but had a > non-null msg_namelen in verify_iovec/verify_compat_iovec. This doesn't > affect sendto as it would bail out earlier while trying to copy-in the > address." > But in fact this affects sendto when address 0 is mapped and contains > socket address structure in it. In such case copy-in address will succeed, > verify_iovec() function will successfully exit with msg->msg_namelen > 0 > and msg->msg_name == NULL. > > This patch fixes it by assigning m->msg_name to address if > move_addr_to_kernel() > was successful. > > Cc: Hannes Frederic Sowa <han...@stressinduktion.org> > Cc: Eric Dumazet <eduma...@google.com> > Cc: <sta...@vger.kernel.org> > Reported-by: Sasha Levin <sasha.le...@oracle.com> > Signed-off-by: Andrey Ryabinin <a.ryabi...@samsung.com> > --- > net/compat.c | 6 ++++-- > net/core/iovec.c | 6 ++++-- > 2 files changed, 8 insertions(+), 4 deletions(-) > > diff --git a/net/compat.c b/net/compat.c > index 9a76eaf..eefd989 100644 > --- a/net/compat.c > +++ b/net/compat.c > @@ -92,9 +92,11 @@ int verify_compat_iovec(struct msghdr *kern_msg, struct > iovec *kern_iov, > kern_address); > if (err < 0) > return err; > - } > - if (kern_msg->msg_name) > + > kern_msg->msg_name = kern_address; > + } else > + if (kern_msg->msg_name) > + kern_msg->msg_name = kern_address; > } else > kern_msg->msg_name = NULL; >
Thanks for looking at this! I certainly have overlooked this case. I wonder, if we allow sendto with valid NULL pointer and positive msg_namelen to work, why don't we do the same for recvmsg, as in replacing the VERIFY_WRITE case non-null check with an access_ok(VERIFY_WRITE, ...) check? We can even get rid of the check because recvmsg will do it nonetheless when copying the sockaddrs to user space. So kern_msg->msg_name can always be set to kern_address. Otherwise I would just set msg_namelen = 0, too, and just not handle passed in NULL pointers to sockaddrs. Thanks, Hannes -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
