From: Johannes Berg <johannes.b...@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 <nat...@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.b...@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
