[PATCH] x86/entry: Fix ptrace regression

[Thomas Gleixner <tglx@kernel.org>]

The recent rework of the syscall entry code missed the historical detail of
the abuse of syscall number -1L to abort a syscall and keep modifications
by tracers to the return value, while a rewrite of the syscall number to an
invalid value undoes eventual tracer modification to the return value and
overwrites it with -ENOSYS.

Restore the original behaviour.

Fixes: 1b1f3b3e1b39 ("x86/entry: Get rid of the sys_ni_syscall() indirection")
Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Closes: https://lore.kernel.org/all/20261022142425.3Wx-RrU5@linutronix.de
---
 arch/x86/entry/syscall_32.c |    2 ++
 arch/x86/entry/syscall_64.c |   26 ++++++++++++++++++++++----
 2 files changed, 24 insertions(+), 4 deletions(-)

--- a/arch/x86/entry/syscall_32.c
+++ b/arch/x86/entry/syscall_32.c
@@ -77,6 +77,8 @@ static __always_inline void do_syscall_3
 	if (likely(nr < IA32_NR_syscalls)) {
 		nr = array_index_nospec(nr, IA32_NR_syscalls);
 		regs->ax = ia32_sys_call(regs, (unsigned int)nr);
+	} else {
+		regs->ax = -ENOSYS;
 	}
 }
 
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -64,15 +64,20 @@ static __always_inline bool do_syscall_x
 	return false;
 }
 
-static __always_inline void do_syscall_x32(struct pt_regs *regs, unsigned long nr)
+static __always_inline bool do_syscall_x32(struct pt_regs *regs, unsigned long nr)
 {
+	if (!IS_ENABLED(CONFIG_X86_X32_ABI))
+		return false;
+
 	/* Adjust the starting offset of the table */
 	nr -= __X32_SYSCALL_BIT;
 
-	if (IS_ENABLED(CONFIG_X86_X32_ABI) && likely(nr < X32_NR_syscalls)) {
+	if (likely(nr < X32_NR_syscalls)) {
 		nr = array_index_nospec(nr, X32_NR_syscalls);
 		regs->ax = x32_sys_call(regs, (unsigned int)nr);
+		return true;
 	}
+	return false;
 }
 
 /* Returns true to return using SYSRET, or false to use IRET */
@@ -81,8 +86,21 @@ static __always_inline void do_syscall_x
 	if (likely(syscall_enter_from_user_mode_randomize_stack(regs, &nr))) {
 		instrumentation_begin();
 
-		if (!do_syscall_x64(regs, nr))
-			do_syscall_x32(regs, nr);
+		if (unlikely(!do_syscall_x64(regs, nr))) {
+			/*
+			 * The special treatment of syscall number -1 is a
+			 * historical bogosity as the original code used -1 to
+			 * indicate syscall abort, which is not distinguishable
+			 * from a syscall which is invoked with syscall number
+			 * -1. The historic behavior is to keep the return value
+			 * in regs->ax unmodified in that case, which means it's
+			 * either the preset value -ENOSYS or a value which was
+			 * written by a tracer. If the syscall number is invalid
+			 * but not -1 then overwrite any tracer modification.
+			 */
+			if (!do_syscall_x32(regs, nr) && nr != -1L)
+				regs->ax = -ENOSYS;
+		}
 
 		instrumentation_end();
 	}