Re: [PATCH] ptrace: zero out siginfo_t in ptrace_peek_siginfo()

[Andrei Vagin]

On Wed, Sep 26, 2018 at 05:17:25PM +0200, Alexander Potapenko wrote:
> KMSAN reported the following infoleak:
> 
> ==================================================================
> BUG: KMSAN: kernel-infoleak in _copy_to_user+0x15d/0x1f0
> ...
> Call Trace:
>  __dump_stack lib/dump_stack.c:77
>  dump_stack+0x2f5/0x430 lib/dump_stack.c:113
>  kmsan_report+0x183/0x2b0 mm/kmsan/kmsan.c:917
>  kmsan_internal_check_memory+0x17e/0x1f0 mm/kmsan/kmsan.c:981
>  kmsan_copy_to_user+0x79/0xc0 mm/kmsan/kmsan_hooks.c:482
>  _copy_to_user+0x15d/0x1f0 lib/usercopy.c:31
>  copy_to_user ./include/linux/uaccess.h:183
>  copy_siginfo_to_user+0x81/0x130 kernel/signal.c:2897
>  ptrace_peek_siginfo kernel/ptrace.c:741
>  ptrace_request+0x2278/0x2680 kernel/ptrace.c:912
>  arch_ptrace+0xbdd/0x11a0 arch/x86/kernel/ptrace.c:877
>  __do_sys_ptrace kernel/ptrace.c:1145
>  __se_sys_ptrace+0x422/0x920 kernel/ptrace.c:1110
>  __x64_sys_ptrace+0x56/0x70 kernel/ptrace.c:1110
>  do_syscall_64+0xb8/0x100 arch/x86/entry/common.c:291
>  entry_SYSCALL_64_after_hwframe+0x63/0xe7 arch/x86/entry/entry_64.S:240
> ...
> Local variable description: ----info.i@ptrace_request
> Variable was created at:
>  ptrace_peek_siginfo kernel/ptrace.c:712
>  ptrace_request+0xdf/0x2680 kernel/ptrace.c:912
>  arch_ptrace+0xbdd/0x11a0 arch/x86/kernel/ptrace.c:877
> 
> Bytes 16-127 of 128 are uninitialized
> Memory access starts at ffff88007af6fc90
> ==================================================================
> 
> when calling ptrace(PTRACE_PEEKSIGINFO) for a traceable child process
> with args = {-1, 0, 1}.
> 
> Initialize the |info| structure to avoid leaking stack data.


"info" is filled up by copy_siginfo(), which overwrites everything.

static inline void copy_siginfo(struct siginfo *to, const struct siginfo *from)
{
        memcpy(to, from, sizeof(*to));
}


so here is another problem. We handle arg.off incorrectly. The right fix
should look something like this:

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 21fec73d45d4..e336434a6f71 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -710,7 +710,7 @@ static int ptrace_peek_siginfo(struct task_struct *child,
 
        for (i = 0; i < arg.nr; ) {
                siginfo_t info;
-               s32 off = arg.off + i;
+               u64 off = arg.off + i;
 
                spin_lock_irq(&child->sighand->siglock);
                list_for_each_entry(q, &pending->list, list) {
@@ -721,7 +721,7 @@ static int ptrace_peek_siginfo(struct task_struct *child,
                }
                spin_unlock_irq(&child->sighand->siglock);
 
-               if (off >= 0) /* beyond the end of the list */
+               if (off + 1 != 0) /* beyond the end of the list */
                        break;
 
 #ifdef CONFIG_COMPAT


> 
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> Reported-by: syzbot+69c3bd9869b32e394c48@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 84c751bd4aebb ("ptrace: add ability to retrieve signals without
> removing from a queue (v4)")
> Cc: Andrey Vagin <avagin@xxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Willy Tarreau <w@xxxxxx>
> ---
>  kernel/ptrace.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 21fec73d45d4..92c3855c2b9c 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -712,6 +712,7 @@ static int ptrace_peek_siginfo(struct task_struct *child,
>  		siginfo_t info;
>  		s32 off = arg.off + i;
>  
> +		memset(&info, 0, sizeof(info));
>  		spin_lock_irq(&child->sighand->siglock);
>  		list_for_each_entry(q, &pending->list, list) {
>  			if (!off--) {
> -- 
> 2.19.0.605.g01d371f741-goog
>