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3ac237fb71
Changed the makecontext logic: previously the first setcontext jumped straight to the user callback function and the return address is set to __startcontext. This does not work when GCS is enabled as the integrity of the return address is protected, so instead the context is setup such that setcontext jumps to __startcontext which calls the user callback (passed in x20). The map_shadow_stack syscall is used to allocate a suitably sized GCS (which includes some reserved area to account for altstack signal handlers and otherwise supports maximum number of 16 byte aligned stack frames on the given stack) however the GCS is never freed as the lifetime of ucontext and related stack is user managed. Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
131 lines
3.7 KiB
C
131 lines
3.7 KiB
C
/* Create new context.
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Copyright (C) 2002-2025 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <sysdep.h>
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#include <stdarg.h>
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#include <stdint.h>
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#include <ucontext.h>
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#include <sys/mman.h>
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#define GCS_MAGIC 0x47435300
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static struct _aarch64_ctx *extension (void *p)
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{
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return p;
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}
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#ifndef SHADOW_STACK_SET_TOKEN
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# define SHADOW_STACK_SET_TOKEN (1UL << 0)
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# define SHADOW_STACK_SET_MARKER (1UL << 1)
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#endif
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static void *
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map_shadow_stack (void *addr, size_t size, unsigned long flags)
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{
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return (void *) INLINE_SYSCALL_CALL (map_shadow_stack, addr, size, flags);
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}
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#define GCS_MAX_SIZE (1UL << 31)
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#define GCS_ALTSTACK_RESERVE 160
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static void *
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alloc_makecontext_gcs (size_t stack_size)
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{
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size_t size = (stack_size / 2 + GCS_ALTSTACK_RESERVE) & -8UL;
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if (size > GCS_MAX_SIZE)
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size = GCS_MAX_SIZE;
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unsigned long flags = SHADOW_STACK_SET_MARKER | SHADOW_STACK_SET_TOKEN;
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void *base = map_shadow_stack (NULL, size, flags);
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if (base == MAP_FAILED)
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/* ENOSYS, bad size or OOM. */
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abort ();
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uint64_t *gcsp = (uint64_t *) ((char *) base + size);
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/* Skip end of GCS token. */
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gcsp--;
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/* Verify GCS cap token. */
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gcsp--;
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if (((uint64_t)gcsp & 0xfffffffffffff000) + 1 != *gcsp)
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abort ();
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/* Return the target GCS pointer for context switch. */
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return gcsp + 1;
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}
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/* makecontext sets up a stack and the registers for the
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user context. The stack looks like this:
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+-----------------------+
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| padding as required |
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+-----------------------+
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sp -> | parameter 7-n |
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+-----------------------+
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The registers are set up like this:
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%x0 .. %x7: parameter 1 to 8
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%x19 : uc_link
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%sp : stack pointer.
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*/
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void
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__makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
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{
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extern void __startcontext (void);
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uint64_t *sp;
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va_list ap;
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int i;
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sp = (uint64_t *)
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((uintptr_t) ucp->uc_stack.ss_sp + ucp->uc_stack.ss_size);
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/* Allocate stack arguments. */
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sp -= argc < 8 ? 0 : argc - 8;
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/* Keep the stack aligned. */
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sp = (uint64_t *) (((uintptr_t) sp) & -16L);
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ucp->uc_mcontext.regs[19] = (uintptr_t) ucp->uc_link;
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ucp->uc_mcontext.regs[20] = (uintptr_t) func;
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ucp->uc_mcontext.sp = (uintptr_t) sp;
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ucp->uc_mcontext.pc = (uintptr_t) __startcontext;
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ucp->uc_mcontext.regs[29] = (uintptr_t) 0;
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ucp->uc_mcontext.regs[30] = (uintptr_t) 0;
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void *p = ucp->uc_mcontext.__reserved;
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if (extension (p)->magic == FPSIMD_MAGIC)
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p = (char *)p + extension (p)->size;
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if (extension (p)->magic == GCS_MAGIC)
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{
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/* Using the kernel struct gcs_context layout. */
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struct { uint64_t x, gcspr, y, z; } *q = p;
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/* TODO: this allocation remains mapped even after thread
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that uses it exits. */
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q->gcspr = (uint64_t) alloc_makecontext_gcs (ucp->uc_stack.ss_size);
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}
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va_start (ap, argc);
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for (i = 0; i < argc; ++i)
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if (i < 8)
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ucp->uc_mcontext.regs[i] = va_arg (ap, uint64_t);
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else
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sp[i - 8] = va_arg (ap, uint64_t);
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va_end (ap);
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}
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weak_alias (__makecontext, makecontext)
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