From 68025adfc13e6cd15eebe2293f77659f47daf13b Mon Sep 17 00:00:00 2001 From: Johannes Berg Date: Fri, 4 Apr 2025 17:05:19 +0200 Subject: 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 Closes: https://lore.kernel.org/r/20250402221254.GA384@ax162 [1] Fixes: d1d7f01f7cd3 ("um: mark rodata read-only and implement _nofault accesses") Tested-by: Nathan Chancellor Signed-off-by: Johannes Berg --- arch/um/include/asm/uaccess.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'arch/um/include/asm') 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) -- cgit v1.2.3