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a09a6e2399
linux/arch/x86/entry/entry_64_compat.S
Peter Zijlstra a09a6e2399 objtool: Add entry UNRET validation 
Since entry asm is tricky, add a validation pass that ensures the
retbleed mitigation has been done before the first actual RET
instruction.

Entry points are those that either have UNWIND_HINT_ENTRY, which acts
as UNWIND_HINT_EMPTY but marks the instruction as an entry point, or
those that have UWIND_HINT_IRET_REGS at +0.

This is basically a variant of validate_branch() that is
intra-function and it will simply follow all branches from marked
entry points and ensures that all paths lead to ANNOTATE_UNRET_END.

If a path hits RET or an indirection the path is a fail and will be
reported.

There are 3 ANNOTATE_UNRET_END instances:

 - UNTRAIN_RET itself
 - exception from-kernel; this path doesn't need UNTRAIN_RET
 - all early exceptions; these also don't need UNTRAIN_RET

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
2022-06-27 10:34:00 +02:00

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/* SPDX-License-Identifier: GPL-2.0 */
/*
* Compatibility mode system call entry point for x86-64.
*
* Copyright 2000-2002 Andi Kleen, SuSE Labs.
*/
#include <asm/asm-offsets.h>
#include <asm/current.h>
#include <asm/errno.h>
#include <asm/ia32_unistd.h>
#include <asm/thread_info.h>
#include <asm/segment.h>
#include <asm/irqflags.h>
#include <asm/asm.h>
#include <asm/smap.h>
#include <asm/nospec-branch.h>
#include <linux/linkage.h>
#include <linux/err.h>
#include "calling.h"
.section .entry.text, "ax"
/*
* 32-bit SYSENTER entry.
*
* 32-bit system calls through the vDSO's __kernel_vsyscall enter here
* on 64-bit kernels running on Intel CPUs.
*
* The SYSENTER instruction, in principle, should *only* occur in the
* vDSO. In practice, a small number of Android devices were shipped
* with a copy of Bionic that inlined a SYSENTER instruction. This
* never happened in any of Google's Bionic versions -- it only happened
* in a narrow range of Intel-provided versions.
*
* SYSENTER loads SS, RSP, CS, and RIP from previously programmed MSRs.
* IF and VM in RFLAGS are cleared (IOW: interrupts are off).
* SYSENTER does not save anything on the stack,
* and does not save old RIP (!!!), RSP, or RFLAGS.
*
* Arguments:
* eax system call number
* ebx arg1
* ecx arg2
* edx arg3
* esi arg4
* edi arg5
* ebp user stack
* 0(%ebp) arg6
*/
SYM_CODE_START(entry_SYSENTER_compat)
UNWIND_HINT_ENTRY
ENDBR
/* Interrupts are off on entry. */
swapgs
pushq %rax
SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
popq %rax
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
/* Construct struct pt_regs on stack */
pushq $__USER32_DS /* pt_regs->ss */
pushq $0 /* pt_regs->sp = 0 (placeholder) */
/*
* Push flags. This is nasty. First, interrupts are currently
* off, but we need pt_regs->flags to have IF set. Second, if TS
* was set in usermode, it's still set, and we're singlestepping
* through this code. do_SYSENTER_32() will fix up IF.
*/
pushfq /* pt_regs->flags (except IF = 0) */
pushq $__USER32_CS /* pt_regs->cs */
pushq $0 /* pt_regs->ip = 0 (placeholder) */
SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
/*
* User tracing code (ptrace or signal handlers) might assume that
* the saved RAX contains a 32-bit number when we're invoking a 32-bit
* syscall. Just in case the high bits are nonzero, zero-extend
* the syscall number. (This could almost certainly be deleted
* with no ill effects.)
*/
movl %eax, %eax
pushq %rax /* pt_regs->orig_ax */
PUSH_AND_CLEAR_REGS rax=$-ENOSYS
UNWIND_HINT_REGS
cld
IBRS_ENTER
UNTRAIN_RET
/*
* SYSENTER doesn't filter flags, so we need to clear NT and AC
* ourselves. To save a few cycles, we can check whether
* either was set instead of doing an unconditional popfq.
* This needs to happen before enabling interrupts so that
* we don't get preempted with NT set.
*
* If TF is set, we will single-step all the way to here -- do_debug
* will ignore all the traps. (Yes, this is slow, but so is
* single-stepping in general. This allows us to avoid having
* a more complicated code to handle the case where a user program
* forces us to single-step through the SYSENTER entry code.)
*
* NB.: .Lsysenter_fix_flags is a label with the code under it moved
* out-of-line as an optimization: NT is unlikely to be set in the
* majority of the cases and instead of polluting the I$ unnecessarily,
* we're keeping that code behind a branch which will predict as
* not-taken and therefore its instructions won't be fetched.
*/
testl $X86_EFLAGS_NT|X86_EFLAGS_AC|X86_EFLAGS_TF, EFLAGS(%rsp)
jnz .Lsysenter_fix_flags
.Lsysenter_flags_fixed:
movq %rsp, %rdi
call do_SYSENTER_32
/* XEN PV guests always use IRET path */
ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
"jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
jmp sysret32_from_system_call
.Lsysenter_fix_flags:
pushq $X86_EFLAGS_FIXED
popfq
jmp .Lsysenter_flags_fixed
SYM_INNER_LABEL(__end_entry_SYSENTER_compat, SYM_L_GLOBAL)
ANNOTATE_NOENDBR // is_sysenter_singlestep
SYM_CODE_END(entry_SYSENTER_compat)
/*
* 32-bit SYSCALL entry.
*
* 32-bit system calls through the vDSO's __kernel_vsyscall enter here
* on 64-bit kernels running on AMD CPUs.
*
* The SYSCALL instruction, in principle, should *only* occur in the
* vDSO. In practice, it appears that this really is the case.
* As evidence:
*
* - The calling convention for SYSCALL has changed several times without
* anyone noticing.
*
* - Prior to the in-kernel X86_BUG_SYSRET_SS_ATTRS fixup, anything
* user task that did SYSCALL without immediately reloading SS
* would randomly crash.
*
* - Most programmers do not directly target AMD CPUs, and the 32-bit
* SYSCALL instruction does not exist on Intel CPUs. Even on AMD
* CPUs, Linux disables the SYSCALL instruction on 32-bit kernels
* because the SYSCALL instruction in legacy/native 32-bit mode (as
* opposed to compat mode) is sufficiently poorly designed as to be
* essentially unusable.
*
* 32-bit SYSCALL saves RIP to RCX, clears RFLAGS.RF, then saves
* RFLAGS to R11, then loads new SS, CS, and RIP from previously
* programmed MSRs. RFLAGS gets masked by a value from another MSR
* (so CLD and CLAC are not needed). SYSCALL does not save anything on
* the stack and does not change RSP.
*
* Note: RFLAGS saving+masking-with-MSR happens only in Long mode
* (in legacy 32-bit mode, IF, RF and VM bits are cleared and that's it).
* Don't get confused: RFLAGS saving+masking depends on Long Mode Active bit
* (EFER.LMA=1), NOT on bitness of userspace where SYSCALL executes
* or target CS descriptor's L bit (SYSCALL does not read segment descriptors).
*
* Arguments:
* eax system call number
* ecx return address
* ebx arg1
* ebp arg2 (note: not saved in the stack frame, should not be touched)
* edx arg3
* esi arg4
* edi arg5
* esp user stack
* 0(%esp) arg6
*/
SYM_CODE_START(entry_SYSCALL_compat)
UNWIND_HINT_ENTRY
ENDBR
/* Interrupts are off on entry. */
swapgs
/* Stash user ESP */
movl %esp, %r8d
/* Use %rsp as scratch reg. User ESP is stashed in r8 */
SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
/* Switch to the kernel stack */
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
SYM_INNER_LABEL(entry_SYSCALL_compat_safe_stack, SYM_L_GLOBAL)
ANNOTATE_NOENDBR
/* Construct struct pt_regs on stack */
pushq $__USER32_DS /* pt_regs->ss */
pushq %r8 /* pt_regs->sp */
pushq %r11 /* pt_regs->flags */
pushq $__USER32_CS /* pt_regs->cs */
pushq %rcx /* pt_regs->ip */
SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL)
movl %eax, %eax /* discard orig_ax high bits */
pushq %rax /* pt_regs->orig_ax */
PUSH_AND_CLEAR_REGS rcx=%rbp rax=$-ENOSYS
UNWIND_HINT_REGS
IBRS_ENTER
UNTRAIN_RET
movq %rsp, %rdi
call do_fast_syscall_32
/* XEN PV guests always use IRET path */
ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
"jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
/* Opportunistic SYSRET */
sysret32_from_system_call:
/*
* We are not going to return to userspace from the trampoline
* stack. So let's erase the thread stack right now.
*/
STACKLEAK_ERASE
IBRS_EXIT
movq RBX(%rsp), %rbx /* pt_regs->rbx */
movq RBP(%rsp), %rbp /* pt_regs->rbp */
movq EFLAGS(%rsp), %r11 /* pt_regs->flags (in r11) */
movq RIP(%rsp), %rcx /* pt_regs->ip (in rcx) */
addq $RAX, %rsp /* Skip r8-r15 */
popq %rax /* pt_regs->rax */
popq %rdx /* Skip pt_regs->cx */
popq %rdx /* pt_regs->dx */
popq %rsi /* pt_regs->si */
popq %rdi /* pt_regs->di */
/*
* USERGS_SYSRET32 does:
* GSBASE = user's GS base
* EIP = ECX
* RFLAGS = R11
* CS = __USER32_CS
* SS = __USER_DS
*
* ECX will not match pt_regs->cx, but we're returning to a vDSO
* trampoline that will fix up RCX, so this is okay.
*
* R12-R15 are callee-saved, so they contain whatever was in them
* when the system call started, which is already known to user
* code. We zero R8-R10 to avoid info leaks.
*/
movq RSP-ORIG_RAX(%rsp), %rsp
SYM_INNER_LABEL(entry_SYSRETL_compat_unsafe_stack, SYM_L_GLOBAL)
ANNOTATE_NOENDBR
/*
* The original userspace %rsp (RSP-ORIG_RAX(%rsp)) is stored
* on the process stack which is not mapped to userspace and
* not readable after we SWITCH_TO_USER_CR3. Delay the CR3
* switch until after after the last reference to the process
* stack.
*
* %r8/%r9 are zeroed before the sysret, thus safe to clobber.
*/
SWITCH_TO_USER_CR3_NOSTACK scratch_reg=%r8 scratch_reg2=%r9
xorl %r8d, %r8d
xorl %r9d, %r9d
xorl %r10d, %r10d
swapgs
sysretl
SYM_INNER_LABEL(entry_SYSRETL_compat_end, SYM_L_GLOBAL)
ANNOTATE_NOENDBR
int3
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
* $0x80 regardless of what instruction was originally used to do the
* system call.
*
* This is considered a slow path. It is not used by most libc
* implementations on modern hardware except during process startup.
*
* Arguments:
* eax system call number
* ebx arg1
* ecx arg2
* edx arg3
* esi arg4
* edi arg5
* ebp arg6
*/
SYM_CODE_START(entry_INT80_compat)
UNWIND_HINT_ENTRY
ENDBR
/*
* Interrupts are off on entry.
*/
ASM_CLAC /* Do this early to minimize exposure */
SWAPGS
/*
* User tracing code (ptrace or signal handlers) might assume that
* the saved RAX contains a 32-bit number when we're invoking a 32-bit
* syscall. Just in case the high bits are nonzero, zero-extend
* the syscall number. (This could almost certainly be deleted
* with no ill effects.)
*/
movl %eax, %eax
/* switch to thread stack expects orig_ax and rdi to be pushed */
pushq %rax /* pt_regs->orig_ax */
/* Need to switch before accessing the thread stack. */
SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
/* In the Xen PV case we already run on the thread stack. */
ALTERNATIVE "", "jmp .Lint80_keep_stack", X86_FEATURE_XENPV
movq %rsp, %rax
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
pushq 5*8(%rax) /* regs->ss */
pushq 4*8(%rax) /* regs->rsp */
pushq 3*8(%rax) /* regs->eflags */
pushq 2*8(%rax) /* regs->cs */
pushq 1*8(%rax) /* regs->ip */
pushq 0*8(%rax) /* regs->orig_ax */
.Lint80_keep_stack:
PUSH_AND_CLEAR_REGS rax=$-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)
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