[PATCH] um: fix _nofault accesses

[Johannes Berg]

From: Johannes Berg <johannes.berg at intel.com>

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 at kernel.org>
Closes: https://lore.kernel.org/r/20250402221254.GA384@ax162 [1]
Fixes: d1d7f01f7cd3 ("um: mark rodata read-only and implement _nofault accesses")
Signed-off-by: Johannes Berg <johannes.berg at intel.com>
---
 arch/um/include/asm/uaccess.h            |  2 ++
 arch/um/kernel/trap.c                    | 26 ++++++++++++------------
 arch/x86/um/shared/sysdep/faultinfo_32.h |  2 +-
 arch/x86/um/shared/sysdep/faultinfo_64.h |  2 +-
 4 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/arch/um/include/asm/uaccess.h b/arch/um/include/asm/uaccess.h
index 3a08f9029a3f..1c6e0ae41b0c 100644
--- a/arch/um/include/asm/uaccess.h
+++ b/arch/um/include/asm/uaccess.h
@@ -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)
 
diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
index ce073150dc20..ef2272e92a43 100644
--- a/arch/um/kernel/trap.c
+++ b/arch/um/kernel/trap.c
@@ -225,20 +225,20 @@ 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) {
-			if (!mc) {
-				show_regs(container_of(regs, struct pt_regs, regs));
-				panic("Segfault with pagefaults disabled but no mcontext");
-			}
-			if (!current->thread.segv_continue) {
-				show_regs(container_of(regs, struct pt_regs, regs));
-				panic("Segfault without recovery target");
-			}
-			mc_set_rip(mc, current->thread.segv_continue);
-			current->thread.segv_continue = NULL;
-			goto out;
+	else if (current->pagefault_disabled) {
+		if (!mc) {
+			show_regs(container_of(regs, struct pt_regs, regs));
+			panic("Segfault with pagefaults disabled but no mcontext");
 		}
+		if (!current->thread.segv_continue) {
+			show_regs(container_of(regs, struct pt_regs, regs));
+			panic("Segfault without recovery target");
+		}
+		mc_set_rip(mc, current->thread.segv_continue);
+		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");
 	}
diff --git a/arch/x86/um/shared/sysdep/faultinfo_32.h b/arch/x86/um/shared/sysdep/faultinfo_32.h
index ab5c8e47049c..9193a7790a71 100644
--- a/arch/x86/um/shared/sysdep/faultinfo_32.h
+++ b/arch/x86/um/shared/sysdep/faultinfo_32.h
@@ -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;"						\
diff --git a/arch/x86/um/shared/sysdep/faultinfo_64.h b/arch/x86/um/shared/sysdep/faultinfo_64.h
index 26fb4835d3e9..61e4ca1e0ab5 100644
--- a/arch/x86/um/shared/sysdep/faultinfo_64.h
+++ b/arch/x86/um/shared/sysdep/faultinfo_64.h
@@ -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;"						\
-- 
2.49.0