2020-06-02 18:09:16

by Dmitry V. Levin

Subject: [PATCH] s390: fix syscall_get_error for compat processes

If both the tracer and the tracee are compat processes, and gprs[2]
is assigned a value by __poke_user_compat, then the higher 32 bits
of gprs[2] are cleared, IS_ERR_VALUE() always returns false, and
syscall_get_error() always returns 0.

Fix the implementation by sign-extending the value for compat processes
the same way as x86 implementation does.

The bug was exposed to user space by commit 201766a20e30f ("ptrace: add
PTRACE_GET_SYSCALL_INFO request") and detected by strace test suite.

This change fixes strace syscall tampering on s390.

Fixes: 753c4dd6a2fa2 ("[S390] ptrace changes")
Cc: Elvira Khabirova <[email protected]>
Cc: [email protected] # v2.6.28+
Signed-off-by: Dmitry V. Levin <[email protected]>
---
arch/s390/include/asm/syscall.h | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/s390/include/asm/syscall.h b/arch/s390/include/asm/syscall.h
index f073292e9fdb..c07633824715 100644
--- a/arch/s390/include/asm/syscall.h
+++ b/arch/s390/include/asm/syscall.h
@@ -33,7 +33,17 @@ static inline void syscall_rollback(struct task_struct *task,
static inline long syscall_get_error(struct task_struct *task,
struct pt_regs *regs)
{
- return IS_ERR_VALUE(regs->gprs[2]) ? regs->gprs[2] : 0;
+ unsigned long error = regs->gprs[2];
+#ifdef CONFIG_COMPAT
+ if (test_tsk_thread_flag(task, TIF_31BIT)) {
+ /*
+ * Sign-extend the value so (int)-EFOO becomes (long)-EFOO
+ * and will match correctly in comparisons.
+ */
+ error = (long) (int) error;
+ }
+#endif
+ return IS_ERR_VALUE(error) ? error : 0;
}

static inline long syscall_get_return_value(struct task_struct *task,
--
ldv

2020-06-16 13:29:48

by Heiko Carstens

Subject: Re: [PATCH] s390: fix syscall_get_error for compat processes

On Tue, Jun 02, 2020 at 09:00:51PM +0300, Dmitry V. Levin wrote:
> If both the tracer and the tracee are compat processes, and gprs[2]
> is assigned a value by __poke_user_compat, then the higher 32 bits
> of gprs[2] are cleared, IS_ERR_VALUE() always returns false, and
> syscall_get_error() always returns 0.
>
> Fix the implementation by sign-extending the value for compat processes
> the same way as x86 implementation does.
>
> The bug was exposed to user space by commit 201766a20e30f ("ptrace: add
> PTRACE_GET_SYSCALL_INFO request") and detected by strace test suite.
>
> This change fixes strace syscall tampering on s390.
>
> Fixes: 753c4dd6a2fa2 ("[S390] ptrace changes")
> Cc: Elvira Khabirova <[email protected]>
> Cc: [email protected] # v2.6.28+
> Signed-off-by: Dmitry V. Levin <[email protected]>
> ---
> arch/s390/include/asm/syscall.h | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)

Applied, thank you!