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s390: fix syscall_get_error for compat processes

PATCH Applied Dmitry V. Levin 2 participants 2 messages Jun 2, 2020 – Jun 16, 2020 s390

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Dmitry V. Levin

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#0 lore raw

PATCH 1/1 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(-)
arch/s390/include/asm/syscall.h +11 -1
index f073292e9fdb..c07633824715 100644
@@ -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,
Signature · 1 line
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#1 lore raw
Quoted 13 lines · Dmitry V. Levin

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.

Quoted 7 lines
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!