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Subject[PATCH v2 06/14] x86/fault: Correct a few user vs kernel checks wrt WRUSS
Date
In general, page fault errors for WRUSS should be just like get_user(),
etc. Fix three bugs in this area:

There is a comment that says that, if the kernel can't handle a page fault
on a user address due to OOM, the OOM-kill-and-retry logic would be
skipped. The code checked kernel *privilege*, not kernel mode, so it
missed WRUSS. This means that the kernel would malfunction if it got OOM
on a WRUSS fault -- this would be a kernel-mode, user-privilege fault, and
the OOM killer would be invoked and the handler would retry the faulting
instruction.

A failed user access from kernel while a fatal signal is pending should
fail even if the instruction in question was WRUSS.

do_sigbus() should not send SIGBUS for WRUSS -- it should handle it like
any other kernel mode failure.

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/mm/fault.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 013910b7b93f..b1104844260d 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -945,7 +945,7 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
vm_fault_t fault)
{
/* Kernel mode? Handle exceptions or die: */
- if (!(error_code & X86_PF_USER)) {
+ if (!user_mode(regs)) {
no_context(regs, error_code, address, SIGBUS, BUS_ADRERR);
return;
}
@@ -1217,7 +1217,14 @@ do_kern_addr_fault(struct pt_regs *regs, unsigned long hw_error_code,
}
NOKPROBE_SYMBOL(do_kern_addr_fault);

-/* Handle faults in the user portion of the address space */
+/*
+ * Handle faults in the user portion of the address space. Nothing in here
+ * should check X86_PF_USER without a specific justification: for almost
+ * all purposes, we should treat a normal kernel access to user memory
+ * (e.g. get_user(), put_user(), etc.) the same as the WRUSS instruction.
+ * The one exception is AC flag handling, which is, per the x86
+ * architecture, special for WRUSS.
+ */
static inline
void do_user_addr_fault(struct pt_regs *regs,
unsigned long error_code,
@@ -1406,14 +1413,14 @@ void do_user_addr_fault(struct pt_regs *regs,
if (likely(!(fault & VM_FAULT_ERROR)))
return;

- if (fatal_signal_pending(current) && !(error_code & X86_PF_USER)) {
+ if (fatal_signal_pending(current) && !user_mode(regs)) {
no_context(regs, error_code, address, 0, 0);
return;
}

if (fault & VM_FAULT_OOM) {
/* Kernel mode? Handle exceptions or die: */
- if (!(error_code & X86_PF_USER)) {
+ if (!user_mode(regs)) {
no_context(regs, error_code, address,
SIGSEGV, SEGV_MAPERR);
return;
--
2.29.2
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