Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations

From: AKASHI Takahiro
Date: Wed Jul 23 2014 - 03:04:03 EST


On 07/23/2014 05:15 AM, Kees Cook wrote:
On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro
<takahiro.akashi@xxxxxxxxxx> wrote:
Arm64 holds a syscall number in w8(x8) register. Ptrace tracer may change
its value either to:
* any valid syscall number to alter a system call, or
* -1 to skip a system call

This patch implements this behavior by reloading that value into syscallno
in struct pt_regs after tracehook_report_syscall_entry() or
secure_computing(). In case of '-1', a return value of system call can also
be changed by the tracer setting the value to x0 register, and so
sys_ni_nosyscall() should not be called.

See also:
42309ab4, ARM: 8087/1: ptrace: reload syscall number after
secure_computing() check

Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
---
arch/arm64/kernel/entry.S | 2 ++
arch/arm64/kernel/ptrace.c | 13 +++++++++++++
2 files changed, 15 insertions(+)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 5141e79..de8bdbc 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -628,6 +628,8 @@ ENDPROC(el0_svc)
__sys_trace:
mov x0, sp
bl syscall_trace_enter
+ cmp w0, #-1 // skip syscall?
+ b.eq ret_to_user
adr lr, __sys_trace_return // return address
uxtw scno, w0 // syscall number (possibly new)
mov x1, sp // pointer to regs
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 70526cf..100d7d1 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -21,6 +21,7 @@

#include <linux/audit.h>
#include <linux/compat.h>
+#include <linux/errno.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/mm.h>
@@ -1109,9 +1110,21 @@ static void tracehook_report_syscall(struct pt_regs *regs,

asmlinkage int syscall_trace_enter(struct pt_regs *regs)
{
+ unsigned long saved_x0, saved_x8;
+
+ saved_x0 = regs->regs[0];
+ saved_x8 = regs->regs[8];
+
if (test_thread_flag(TIF_SYSCALL_TRACE))
tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);

+ regs->syscallno = regs->regs[8];
+ if ((long)regs->syscallno == ~0UL) { /* skip this syscall */
+ regs->regs[8] = saved_x8;
+ if (regs->regs[0] == saved_x0) /* not changed by user */
+ regs->regs[0] = -ENOSYS;

I'm not sure this is right compared to other architectures. Generally
when a tracer performs a syscall skip, it's up to them to also adjust
the return value. They may want to be faking a syscall, and what if
the value they want to return happens to be what x0 was going into the
tracer? It would have no way to avoid this -ENOSYS case. I think
things are fine without this test.

Yeah, I know this issue, but was not sure that setting a return value
is mandatory. (x86 seems to return -ENOSYS by default if not explicitly
specified.)
Is "fake a system call" a more appropriate word than "skip"?

I will defer to Will.

Thanks,
-Takahiro AKASHI

-Kees

+ }
+
if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
trace_sys_enter(regs, regs->syscallno);

--
1.7.9.5




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