From 331955600ddf55a2c6d92a00f95b0865f1c74fc3 Mon Sep 17 00:00:00 2001 From: Rick Edgecombe Date: Fri, 8 Sep 2023 13:36:53 -0700 Subject: [PATCH] x86/shstk: Handle vfork clone failure correctly Shadow stacks are allocated automatically and freed on exit, depending on the clone flags. The two cases where new shadow stacks are not allocated are !CLONE_VM (fork()) and CLONE_VFORK (vfork()). For !CLONE_VM, although a new stack is not allocated, it can be freed normally because it will happen in the child's copy of the VM. However, for CLONE_VFORK the parent and the child are actually using the same shadow stack. So the kernel doesn't need to allocate *or* free a shadow stack for a CLONE_VFORK child. CLONE_VFORK children already need special tracking to avoid returning to userspace until the child exits or execs. Shadow stack uses this same tracking to avoid freeing CLONE_VFORK shadow stacks. However, the tracking is not setup until the clone has succeeded (internally). Which means, if a CLONE_VFORK fails, the existing logic will not know it is a CLONE_VFORK and proceed to unmap the parents shadow stack. This error handling cleanup logic runs via exit_thread() in the bad_fork_cleanup_thread label in copy_process(). The issue was seen in the glibc test "posix/tst-spawn3-pidfd" while running with shadow stack using currently out-of-tree glibc patches. Fix it by not unmapping the vfork shadow stack in the error case as well. Since clone is implemented in core code, it is not ideal to pass the clone flags along the error path in order to have shadow stack code have symmetric logic in the freeing half of the thread shadow stack handling. Instead use the existing state for thread shadow stacks to track whether the thread is managing its own shadow stack. For CLONE_VFORK, simply set shstk->base and shstk->size to 0, and have it mean the thread is not managing a shadow stack and so should skip cleanup work. Implement this by breaking up the CLONE_VFORK and !CLONE_VM cases in shstk_alloc_thread_stack() to separate conditionals since, the logic is now different between them. In the case of CLONE_VFORK && !CLONE_VM, the existing behavior is to not clean up the shadow stack in the child (which should go away quickly with either be exit or exec), so maintain that behavior by handling the CLONE_VFORK case first in the allocation path. This new logioc cleanly handles the case of normal, successful CLONE_VFORK's skipping cleaning up their shadow stack's on exit as well. So remove the existing, vfork shadow stack freeing logic. This is in deactivate_mm() where vfork_done is used to tell if it is a vfork child that can skip cleaning up the thread shadow stack. Fixes: b2926a36b97a ("x86/shstk: Handle thread shadow stack") Reported-by: H.J. Lu Signed-off-by: Rick Edgecombe Signed-off-by: Dave Hansen Tested-by: H.J. Lu Link: https://lore.kernel.org/all/20230908203655.543765-2-rick.p.edgecombe%40intel.com --- arch/x86/include/asm/mmu_context.h | 3 +-- arch/x86/kernel/shstk.c | 22 ++++++++++++++++++++-- 2 files changed, 21 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h index 416901d..8dac45a 100644 --- a/arch/x86/include/asm/mmu_context.h +++ b/arch/x86/include/asm/mmu_context.h @@ -186,8 +186,7 @@ do { \ #else #define deactivate_mm(tsk, mm) \ do { \ - if (!tsk->vfork_done) \ - shstk_free(tsk); \ + shstk_free(tsk); \ load_gs_index(0); \ loadsegment(fs, 0); \ } while (0) diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c index fd68992..ad63252 100644 --- a/arch/x86/kernel/shstk.c +++ b/arch/x86/kernel/shstk.c @@ -205,10 +205,21 @@ unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long cl return 0; /* - * For CLONE_VM, except vfork, the child needs a separate shadow + * For CLONE_VFORK the child will share the parents shadow stack. + * Make sure to clear the internal tracking of the thread shadow + * stack so the freeing logic run for child knows to leave it alone. + */ + if (clone_flags & CLONE_VFORK) { + shstk->base = 0; + shstk->size = 0; + return 0; + } + + /* + * For !CLONE_VM the child will use a copy of the parents shadow * stack. */ - if ((clone_flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM) + if (!(clone_flags & CLONE_VM)) return 0; size = adjust_shstk_size(stack_size); @@ -408,6 +419,13 @@ void shstk_free(struct task_struct *tsk) if (!tsk->mm || tsk->mm != current->mm) return; + /* + * If shstk->base is NULL, then this task is not managing its + * own shadow stack (CLONE_VFORK). So skip freeing it. + */ + if (!shstk->base) + return; + unmap_shadow_stack(shstk->base, shstk->size); } -- 2.7.4