Commit 68025adf authored Apr 04, 2025 by Johannes Berg

um: fix _nofault accesses



Nathan reported [1] that when built with clang, the um kernel
crashes pretty much immediately. This turned out to be an issue
with the inline assembly I had added, when clang used %rax/%eax
for both operands. Reorder it so current->thread.segv_continue
is written first, and then the lifetime of _faulted won't have
overlap with the lifetime of segv_continue.

In the email thread Benjamin also pointed out that current->mm
is only NULL for true kernel tasks, but we could do this for a
userspace task, so the current->thread.segv_continue logic must
be lifted out of the mm==NULL check.

Finally, while looking at this, put a barrier() so the NULL
assignment to thread.segv_continue cannot be reorder before
the possibly faulting operation.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/r/20250402221254.GA384@ax162

 [1]
Fixes: d1d7f01f ("um: mark rodata read-only and implement _nofault accesses")
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

parent 92a09c47

arch/um/include/asm/uaccess.h

+2 −0

Original line number	Diff line number	Diff line
		@@ -55,6 +55,7 @@ do { \
		goto err_label; \
		} \
		((type )dst) = get_unaligned((type *)(src)); \
		barrier(); \
		current->thread.segv_continue = NULL; \
		} while (0)

		@@ -66,6 +67,7 @@ do { \
		if (__faulted) \
		goto err_label; \
		put_unaligned(((type )src), (type *)(dst)); \
		barrier(); \
		current->thread.segv_continue = NULL; \
		} while (0)

arch/um/kernel/trap.c

+13 −13

Original line number	Diff line number	Diff line
		@@ -225,8 +225,7 @@ unsigned long segv(struct faultinfo fi, unsigned long ip, int is_user,
		panic("Failed to sync kernel TLBs: %d", err);
		goto out;
		}
		else if (current->mm == NULL) {
		if (current->pagefault_disabled) {
		else if (current->pagefault_disabled) {
		if (!mc) {
		show_regs(container_of(regs, struct pt_regs, regs));
		panic("Segfault with pagefaults disabled but no mcontext");
		@@ -239,6 +238,7 @@ unsigned long segv(struct faultinfo fi, unsigned long ip, int is_user,
		current->thread.segv_continue = NULL;
		goto out;
		}
		else if (current->mm == NULL) {
		show_regs(container_of(regs, struct pt_regs, regs));
		panic("Segfault with no mm");
		}

arch/x86/um/shared/sysdep/faultinfo_32.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -31,8 +31,8 @@ struct faultinfo {

		#define ___backtrack_faulted(_faulted) \
		asm volatile ( \
		"mov $0, %0\n" \
		"movl $__get_kernel_nofault_faulted_%=,%1\n" \
		"mov $0, %0\n" \
		"jmp _end_%=\n" \
		"__get_kernel_nofault_faulted_%=:\n" \
		"mov $1, %0;" \

arch/x86/um/shared/sysdep/faultinfo_64.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -31,8 +31,8 @@ struct faultinfo {

		#define ___backtrack_faulted(_faulted) \
		asm volatile ( \
		"mov $0, %0\n" \
		"movq $__get_kernel_nofault_faulted_%=,%1\n" \
		"mov $0, %0\n" \
		"jmp _end_%=\n" \
		"__get_kernel_nofault_faulted_%=:\n" \
		"mov $1, %0;" \