On Sat, Jul 14, 2012 at 10:44 AM, Andrew Lutomirski <luto@xxxxxxx> wrote:
> I think I'd prefer if changing to something other than whatever value is
> used to cancel the syscall resulted in a crash rather than just being
> ignored.
I was trying to keep as much seccomp-ptrace behavior intact rather
than making it terminal in this special case. Is there a reason why
it'd make more sense to crash?
> How hard is it for a page fault to return into the syscall entry path? It
> should be possible to do this for rel, although it could be messy and not
> worth it.
Not sure, tbh. I think given vsyscall's status and the fact that
ptrace+seccomp+vsyscall=emulate isn't horrible, I think it's fine to
either ignore (what is in tree now) or to allow ptrace to skip,
without providing full functionality. But obviously, my view my be
biased!
thanks!
will
>
> On Jul 14, 2012 10:35 AM, "Will Drewry" <wad@xxxxxxxxxxxx> wrote:
>>
>> Current quirky ptrace behavior with vsyscall and seccomp
>> does not allow tracers to bypass the call. This change
>> provides that ability by checking if orig_ax changed.
>>
>> Signed-off-by: Will Drewry <wad@xxxxxxxxxxxx>
>> ---
>> arch/x86/kernel/vsyscall_64.c | 10 +++++++---
>> 1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
>> index 5db36ca..5f9640c 100644
>> --- a/arch/x86/kernel/vsyscall_64.c
>> +++ b/arch/x86/kernel/vsyscall_64.c
>> @@ -142,11 +142,15 @@ static int addr_to_vsyscall_nr(unsigned long addr)
>> #ifdef CONFIG_SECCOMP
>> static int vsyscall_seccomp(struct task_struct *tsk, int syscall_nr)
>> {
>> + int ret;
>> if (!seccomp_mode(&tsk->seccomp))
>> return 0;
>> task_pt_regs(tsk)->orig_ax = syscall_nr;
>> task_pt_regs(tsk)->ax = syscall_nr;
>> - return __secure_computing(syscall_nr);
>> + ret = __secure_computing(syscall_nr);
>> + if (task_pt_regs(tsk)->orig_ax != syscall_nr)
>> + return 1; /* ptrace syscall skip */
>> + return ret;
>> }
>> #else
>> #define vsyscall_seccomp(_tsk, _nr) 0
>> @@ -278,9 +282,9 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned
>> long address)
>> current_thread_info()->sig_on_uaccess_error =
>> prev_sig_on_uaccess_error;
>>
>> if (skip) {
>> - if ((long)regs->ax <= 0L) /* seccomp errno emulation */
>> + if ((long)regs->ax <= 0L || skip == 1) /* seccomp
>> errno/trace */
>> goto do_ret;
>> - goto done; /* seccomp trace/trap */
>> + goto done; /* seccomp trap */
>> }
>>
>> if (ret == -EFAULT) {
>> --
>> 1.7.9.5
>>
>
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