x86/entry: Convert INT 0x80 emulation to IDTENTRY

jira VULN-772
cve CVE-2024-25744
commit-author Thomas Gleixner <tglx@linutronix.de>
commit be5341e

There is no real reason to have a separate ASM entry point implementation
for the legacy INT 0x80 syscall emulation on 64-bit.

IDTENTRY provides all the functionality needed with the only difference
that it does not:

  - save the syscall number (AX) into pt_regs::orig_ax
  - set pt_regs::ax to -ENOSYS

Both can be done safely in the C code of an IDTENTRY before invoking any of
the syscall related functions which depend on this convention.

Aside of ASM code reduction this prepares for detecting and handling a
local APIC injected vector 0x80.

[ kirill.shutemov: More verbose comments ]
	Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
	Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
	Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
	Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
	Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
	Cc: <stable@vger.kernel.org> # v6.0+
(cherry picked from commit be5341e)
	Signed-off-by: Marcin Wcisło <marcin.wcislo@conclusive.pl>

Author: pvts-mat

arch/x86/entry/common.c

@@ -126,7 +126,62 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs, int nr)
 	}
 }
 
-/* Handles int $0x80 */
+#ifdef CONFIG_IA32_EMULATION
+/**
+ * int80_emulation - 32-bit legacy syscall entry
+ *
+ * This entry point can be used by 32-bit and 64-bit programs to perform
+ * 32-bit system calls.  Instances of INT $0x80 can be found inline in
+ * various programs and libraries.  It is also used by the vDSO's
+ * __kernel_vsyscall fallback for hardware that doesn't support a faster
+ * entry method.  Restarted 32-bit system calls also fall back to INT
+ * $0x80 regardless of what instruction was originally used to do the
+ * system call.
+ *
+ * This is considered a slow path.  It is not used by most libc
+ * implementations on modern hardware except during process startup.
+ *
+ * The arguments for the INT $0x80 based syscall are on stack in the
+ * pt_regs structure:
+ *   eax:				system call number
+ *   ebx, ecx, edx, esi, edi, ebp:	arg1 - arg 6
+ */
+DEFINE_IDTENTRY_RAW(int80_emulation)
+{
+	int nr;
+
+	/* Establish kernel context. */
+	enter_from_user_mode(regs);
+
+	instrumentation_begin();
+	add_random_kstack_offset();
+
+	/*
+	 * The low level idtentry code pushed -1 into regs::orig_ax
+	 * and regs::ax contains the syscall number.
+	 *
+	 * User tracing code (ptrace or signal handlers) might assume
+	 * that the regs::orig_ax contains a 32-bit number on invoking
+	 * a 32-bit syscall.
+	 *
+	 * Establish the syscall convention by saving the 32bit truncated
+	 * syscall number in regs::orig_ax and by invalidating regs::ax.
+	 */
+	regs->orig_ax = regs->ax & GENMASK(31, 0);
+	regs->ax = -ENOSYS;
+
+	nr = syscall_32_enter(regs);
+
+	local_irq_enable();
+	nr = syscall_enter_from_user_mode_work(regs, nr);
+	do_syscall_32_irqs_on(regs, nr);
+
+	instrumentation_end();
+	syscall_exit_to_user_mode(regs);
+}
+#else /* CONFIG_IA32_EMULATION */
+
+/* Handles int $0x80 on a 32bit kernel */
 __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
 {
 	int nr = syscall_32_enter(regs);
@@ -145,6 +200,7 @@ __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
 	instrumentation_end();
 	syscall_exit_to_user_mode(regs);
 }
+#endif /* !CONFIG_IA32_EMULATION */
 
 static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
 {

arch/x86/entry/entry_64_compat.S

@@ -274,80 +274,3 @@ SYM_INNER_LABEL(entry_SYSRETL_compat_unsafe_stack, SYM_L_GLOBAL)
 	sysretl
 SYM_INNER_LABEL(entry_SYSRETL_compat_end, SYM_L_GLOBAL)
 SYM_CODE_END(entry_SYSCALL_compat)
-
-/*
- * 32-bit legacy system call entry.
- *
- * 32-bit x86 Linux system calls traditionally used the INT $0x80
- * instruction.  INT $0x80 lands here.
- *
- * This entry point can be used by 32-bit and 64-bit programs to perform
- * 32-bit system calls.  Instances of INT $0x80 can be found inline in
- * various programs and libraries.  It is also used by the vDSO's
- * __kernel_vsyscall fallback for hardware that doesn't support a faster
- * entry method.  Restarted 32-bit system calls also fall back to INT
- * $0x80 regardless of what instruction was originally used to do the
- * system call.
- *
- * This is considered a slow path.  It is not used by most libc
- * implementations on modern hardware except during process startup.
- *
- * Arguments:
- * eax  system call number
- * ebx  arg1
- * ecx  arg2
- * edx  arg3
- * esi  arg4
- * edi  arg5
- * ebp  arg6
- */
-SYM_CODE_START(entry_INT80_compat)
-	UNWIND_HINT_EMPTY
-	ENDBR
-	/*
-	 * Interrupts are off on entry.
-	 */
-	ASM_CLAC			/* Do this early to minimize exposure */
-	SWAPGS
-
-	/*
-	 * User tracing code (ptrace or signal handlers) might assume that
-	 * the saved RAX contains a 32-bit number when we're invoking a 32-bit
-	 * syscall.  Just in case the high bits are nonzero, zero-extend
-	 * the syscall number.  (This could almost certainly be deleted
-	 * with no ill effects.)
-	 */
-	movl	%eax, %eax
-
-	/* switch to thread stack expects orig_ax and rdi to be pushed */
-	pushq	%rax			/* pt_regs->orig_ax */
-
-	/* Need to switch before accessing the thread stack. */
-	SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
-
-	/* In the Xen PV case we already run on the thread stack. */
-	ALTERNATIVE "", "jmp .Lint80_keep_stack", X86_FEATURE_XENPV
-
-	movq	%rsp, %rax
-	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
-
-	pushq	5*8(%rax)		/* regs->ss */
-	pushq	4*8(%rax)		/* regs->rsp */
-	pushq	3*8(%rax)		/* regs->eflags */
-	pushq	2*8(%rax)		/* regs->cs */
-	pushq	1*8(%rax)		/* regs->ip */
-	pushq	0*8(%rax)		/* regs->orig_ax */
-.Lint80_keep_stack:
-
-	PUSH_AND_CLEAR_REGS rax=$-ENOSYS
-	UNWIND_HINT_REGS
-
-	cld
-
-	IBRS_ENTER
-	UNTRAIN_RET
-
-	movq	%rsp, %rdi
-	call	do_int80_syscall_32
-	jmp	swapgs_restore_regs_and_return_to_usermode
-SYM_CODE_END(entry_INT80_compat)

arch/x86/include/asm/idtentry.h

@@ -569,6 +569,10 @@ DECLARE_IDTENTRY_RAW(X86_TRAP_UD,		exc_invalid_op);
 DECLARE_IDTENTRY_RAW(X86_TRAP_BP,		exc_int3);
 DECLARE_IDTENTRY_RAW_ERRORCODE(X86_TRAP_PF,	exc_page_fault);
 
+#if defined(CONFIG_IA32_EMULATION)
+DECLARE_IDTENTRY_RAW(IA32_SYSCALL_VECTOR,	int80_emulation);
+#endif
+
 #ifdef CONFIG_X86_MCE
 #ifdef CONFIG_X86_64
 DECLARE_IDTENTRY_MCE(X86_TRAP_MC,	exc_machine_check);

arch/x86/include/asm/proto.h

@@ -32,10 +32,6 @@ void entry_SYSCALL_compat(void);
 void entry_SYSCALL_compat_safe_stack(void);
 void entry_SYSRETL_compat_unsafe_stack(void);
 void entry_SYSRETL_compat_end(void);
-void entry_INT80_compat(void);
-#ifdef CONFIG_XEN_PV
-void xen_entry_INT80_compat(void);
-#endif
 #else /* !CONFIG_IA32_EMULATION */
 #define entry_SYSCALL_compat NULL
 #define entry_SYSENTER_compat NULL

arch/x86/kernel/idt.c

@@ -121,7 +121,7 @@ static const __initconst struct idt_data def_idts[] = {
 
 static const struct idt_data ia32_idt[] __initconst = {
 #if defined(CONFIG_IA32_EMULATION)
-	SYSG(IA32_SYSCALL_VECTOR,	entry_INT80_compat),
+	SYSG(IA32_SYSCALL_VECTOR,	asm_int80_emulation),
 #elif defined(CONFIG_X86_32)
 	SYSG(IA32_SYSCALL_VECTOR,	entry_INT80_32),
 #endif

arch/x86/xen/enlighten_pv.c

@@ -637,7 +637,7 @@ static struct trap_array_entry trap_array[] = {
 	TRAP_ENTRY(exc_int3,				false ),
 	TRAP_ENTRY(exc_overflow,			false ),
 #ifdef CONFIG_IA32_EMULATION
-	{ entry_INT80_compat,          xen_entry_INT80_compat,          false },
+	TRAP_ENTRY(int80_emulation,			false ),
 #endif
 	TRAP_ENTRY(exc_page_fault,			false ),
 	TRAP_ENTRY(exc_divide_error,			false ),

arch/x86/xen/xen-asm.S

@@ -156,7 +156,7 @@ xen_pv_trap asm_xenpv_exc_machine_check
 #endif /* CONFIG_X86_MCE */
 xen_pv_trap asm_exc_simd_coprocessor_error
 #ifdef CONFIG_IA32_EMULATION
-xen_pv_trap entry_INT80_compat
+xen_pv_trap asm_int80_emulation
 #endif
 xen_pv_trap asm_exc_xen_unknown_trap
 xen_pv_trap asm_exc_xen_hypervisor_callback