On 07/25/2014 08:03 PM, Will Deacon wrote:
On Fri, Jul 25, 2014 at 11:36:49AM +0100, AKASHI Takahiro wrote:
On 07/25/2014 12:01 AM, Andy Lutomirski wrote:
If so, then you risk (at least) introducing
a nice user-triggerable OOPS if audit is enabled.
Can you please elaborate this? Since I didn't find any definition of audit's behavior when syscall is rewritten to -1, I thought it is reasonable to skip "exit tracing" of "skipped" syscall. (otherwise, "fake" seems to be more appropriate :)
The audit entry hook will oops if you call it twice in a row without calling the exit hook in between.
Thank you, I could reproduce this problem which hits BUG(in_syscall) in audit_syscall_entry(). Really bad, and I fixed it in my next version and now a "skipped" system call is also traced by audit.
Can you reproduce this on arch/arm/ too? If so, we should also fix the code there.
As far as I tried on arm with syscall auditing enabled,
1) Changing a syscall number to -1 under seccomp doesn't hit BUG_ON(in_syscall). 2) But, in fact, audit_syscall_entry() is NOT called in this case because __secure_computing() returns -1 and then it causes the succeeding tracing in syscall_trace_enter(), including audit_syscall_entry(), skipped. 3) On the other hand, calling syscall(-1) from userspace hits BUG_ON because the return path, ret_slow_syscall, doesn't contain syscall_trace_exit(). 4) When we re-write a syscall number to -1 without seccomp, we will also see BUG_ON hit, although I didn't try yet.
Fixing case 3 is easy, but should we also fix case 2?
Please note that, even if we call audit_syscall_exit() in case 2 or 3, no log against syscall -1 will be recorded because audit_filter_syscall() doesn't allow logging for any syscall number which is greater than 2048. This behavior was introduced by Andy's patch, a3c54931, in v3.16-rc. If the intention of "-1" is to fake a system call, this behavior seems to be a bit odd.
Thanks, -Takahiro AKASHI
Will