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68025adfc1
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: d1d7f01f7c ("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>
44 lines
1.3 KiB
C
44 lines
1.3 KiB
C
/*
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* Copyright (C) 2004 Fujitsu Siemens Computers GmbH
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* Author: Bodo Stroesser <bstroesser@fujitsu-siemens.com>
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* Licensed under the GPL
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*/
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#ifndef __FAULTINFO_I386_H
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#define __FAULTINFO_I386_H
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/* this structure contains the full arch-specific faultinfo
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* from the traps.
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* On i386, ptrace_faultinfo unfortunately doesn't provide
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* all the info, since trap_no is missing.
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* All common elements are defined at the same position in
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* both structures, thus making it easy to copy the
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* contents without knowledge about the structure elements.
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*/
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struct faultinfo {
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int error_code; /* in ptrace_faultinfo misleadingly called is_write */
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unsigned long cr2; /* in ptrace_faultinfo called addr */
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int trap_no; /* missing in ptrace_faultinfo */
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};
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#define FAULT_WRITE(fi) ((fi).error_code & 2)
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#define FAULT_ADDRESS(fi) ((fi).cr2)
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/* This is Page Fault */
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#define SEGV_IS_FIXABLE(fi) ((fi)->trap_no == 14)
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#define PTRACE_FULL_FAULTINFO 0
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#define ___backtrack_faulted(_faulted) \
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asm volatile ( \
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"movl $__get_kernel_nofault_faulted_%=,%1\n" \
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"mov $0, %0\n" \
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"jmp _end_%=\n" \
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"__get_kernel_nofault_faulted_%=:\n" \
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"mov $1, %0;" \
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"_end_%=:" \
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: "=r" (_faulted), \
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"=m" (current->thread.segv_continue) :: \
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)
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#endif
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