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glibc/sysdeps/unix/sysv/linux/i386/swapcontext.S
H.J. Lu 5d844e1b72 i386: Enable CET support in ucontext functions 
1. getcontext and swapcontext are updated to save the caller's shadow
stack pointer and return address.
2. setcontext and swapcontext are updated to restore shadow stack and
jump to new context directly.
3. makecontext is updated to allocate a new shadow stack and set the
caller's return address to the helper code, L(exitcode).
4. Since we no longer save and restore EAX, ECX and EDX in getcontext,
setcontext and swapcontext, we can use them as scratch register slots
to enable CET in ucontext functions.

Since makecontext allocates a new shadow stack when making a new
context and kernel allocates a new shadow stack for clone/fork/vfork
syscalls, we track the current shadow stack base.  In setcontext and
swapcontext, if the target shadow stack base is the same as the current
shadow stack base, we unwind the shadow stack.  Otherwise it is a stack
switch and we look for a restore token.

We enable shadow stack at run-time only if program and all used shared
objects, including dlopened ones, are shadow stack enabled, which means
that they must be compiled with GCC 8 or above and glibc 2.28 or above.
We need to save and restore shadow stack only if shadow stack is enabled.
When caller of getcontext, setcontext, swapcontext and makecontext is
compiled with smaller ucontext_t, shadow stack won't be enabled at
run-time.  We check if shadow stack is enabled before accessing the
extended field in ucontext_t.

Tested on i386 CET/non-CET machines.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
2020-02-14 15:15:25 -08:00

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/* Save current context and install the given one.
Copyright (C) 2001-2020 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Ulrich Drepper <drepper@redhat.com>, 2001.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <sysdep.h>
#include <asm/prctl.h>
#include "ucontext_i.h"
ENTRY(__swapcontext)
/* Load address of the context data structure we save in. */
movl 4(%esp), %eax
/* Save the preserved register values and the return address. */
movl %edi, oEDI(%eax)
movl %esi, oESI(%eax)
movl %ebp, oEBP(%eax)
movl (%esp), %ecx
movl %ecx, oEIP(%eax)
leal 4(%esp), %ecx
movl %ecx, oESP(%eax)
movl %ebx, oEBX(%eax)
/* Save the FS segment register. */
xorl %edx, %edx
movw %fs, %dx
movl %edx, oFS(%eax)
/* We have separate floating-point register content memory on the
stack. We use the __fpregs_mem block in the context. Set the
links up correctly. */
leal oFPREGSMEM(%eax), %ecx
movl %ecx, oFPREGS(%eax)
/* Save the floating-point context. */
fnstenv (%ecx)
/* Load address of the context data structure we have to load. */
movl 8(%esp), %ecx
/* Save the current signal mask and install the new one. */
pushl %ebx
leal oSIGMASK(%eax), %edx
leal oSIGMASK(%ecx), %ecx
movl $SIG_SETMASK, %ebx
movl $__NR_sigprocmask, %eax
ENTER_KERNEL
popl %ebx
cmpl $-4095, %eax /* Check %eax for error. */
jae SYSCALL_ERROR_LABEL /* Jump to error handler if error. */
/* EAX was modified, reload it. */
movl 8(%esp), %eax
/* Restore the floating-point context. Not the registers, only the
rest. */
movl oFPREGS(%eax), %ecx
fldenv (%ecx)
/* Restore the FS segment register. We don't touch the GS register
since it is used for threads. */
movl oFS(%eax), %edx
movw %dx, %fs
#if SHSTK_ENABLED
/* Check if Shadow Stack is enabled. */
testl $X86_FEATURE_1_SHSTK, %gs:FEATURE_1_OFFSET
jz L(no_shstk)
xorl %eax, %eax
cmpl %gs:SSP_BASE_OFFSET, %eax
jnz L(shadow_stack_bound_recorded)
/* Get the base address and size of the default shadow stack
which must be the current shadow stack since nothing has
been recorded yet. */
sub $24, %esp
mov %esp, %ecx
movl $ARCH_CET_STATUS, %ebx
movl $__NR_arch_prctl, %eax
ENTER_KERNEL
testl %eax, %eax
jz L(continue_no_err)
/* This should never happen. */
hlt
L(continue_no_err):
/* Record the base of the current shadow stack. */
movl 8(%esp), %eax
movl %eax, %gs:SSP_BASE_OFFSET
add $24, %esp
L(shadow_stack_bound_recorded):
/* Load address of the context data structure we save in. */
movl 4(%esp), %eax
/* Load address of the context data structure we swap in */
movl 8(%esp), %edx
/* If we unwind the stack, we can't undo stack unwinding. Just
save the target shadow stack pointer as the current shadow
stack pointer. */
movl oSSP(%edx), %ecx
movl %ecx, oSSP(%eax)
/* Save the current shadow stack base in ucontext. */
movl %gs:SSP_BASE_OFFSET, %ecx
movl %ecx, (oSSP + 4)(%eax)
/* If the base of the target shadow stack is the same as the
base of the current shadow stack, we unwind the shadow
stack. Otherwise it is a stack switch and we look for a
restore token. */
movl oSSP(%edx), %esi
movl %esi, %edi
/* Get the base of the target shadow stack. */
movl (oSSP + 4)(%edx), %ecx
cmpl %gs:SSP_BASE_OFFSET, %ecx
je L(unwind_shadow_stack)
/* Align the saved original shadow stack pointer to the next
8 byte aligned boundary. */
andl $-8, %esi
L(find_restore_token_loop):
/* Look for a restore token. */
movl -8(%esi), %ebx
andl $-8, %ebx
cmpl %esi, %ebx
je L(restore_shadow_stack)
/* Try the next slot. */
subl $8, %esi
jmp L(find_restore_token_loop)
L(restore_shadow_stack):
/* The target shadow stack will be restored. Save the current
shadow stack pointer. */
rdsspd %ecx
movl %ecx, oSSP(%eax)
/* Use the restore stoken to restore the target shadow stack. */
rstorssp -8(%esi)
/* Save the restore token on the old shadow stack. NB: This
restore token may be checked by setcontext or swapcontext
later. */
saveprevssp
/* Record the new shadow stack base that was switched to. */
movl (oSSP + 4)(%edx), %ebx
movl %ebx, %gs:SSP_BASE_OFFSET
L(unwind_shadow_stack):
rdsspd %ebx
subl %edi, %ebx
je L(skip_unwind_shadow_stack)
negl %ebx
shrl $2, %ebx
movl $255, %esi
L(loop):
cmpl %esi, %ebx
cmovb %ebx, %esi
incsspd %esi
subl %esi, %ebx
ja L(loop)
L(skip_unwind_shadow_stack):
/* Load the new stack pointer. */
movl oESP(%edx), %esp
/* Load the values of all the preserved registers (except ESP). */
movl oEDI(%edx), %edi
movl oESI(%edx), %esi
movl oEBP(%edx), %ebp
movl oEBX(%edx), %ebx
/* Get the return address set with getcontext. */
movl oEIP(%edx), %ecx
/* Check if return address is valid for the case when setcontext
is invoked from L(exitcode) with linked context. */
rdsspd %eax
cmpl (%eax), %ecx
/* Clear EAX to indicate success. NB: Don't use xorl to keep
EFLAGS for jne. */
movl $0, %eax
jne L(jmp)
/* Return to the new context if return address valid. */
pushl %ecx
ret
L(jmp):
/* Jump to the new context directly. */
jmp *%ecx
L(no_shstk):
#endif
/* Fetch the address to return to. */
movl oEIP(%eax), %ecx
/* Load the new stack pointer. */
movl oESP(%eax), %esp
/* Push the return address on the new stack so we can return there. */
pushl %ecx
/* Load the values of all the preserved registers (except ESP). */
movl oEDI(%eax), %edi
movl oESI(%eax), %esi
movl oEBP(%eax), %ebp
movl oEBX(%eax), %ebx
/* All done, return 0 for success. */
xorl %eax, %eax
/* The following 'ret' will pop the address of the code and jump
to it. */
ret
PSEUDO_END(__swapcontext)
weak_alias (__swapcontext, swapcontext)
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