Re: [RFC PATCH -next V3 4/6] arm64: add copy_{to, from}_user to machine check safe

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On 12/04/2022 6:08 pm, Robin Murphy wrote:
[...]
@@ -62,7 +63,11 @@ SYM_FUNC_START(__arch_copy_from_user)
      ret
      // Exception fixups
-9997:    cmp    dst, dstin
+9997:    mrs esr, esr_el1            // Check exception first
+    and esr, esr, #ESR_ELx_FSC
+    cmp esr, #ESR_ELx_FSC_EXTABT

Should we be checking EC to make sure it's a data abort - and thus FSC is valid - in the first place? I'm a little fuzzy on all the possible paths into fixup_exception(), and it's not entirely obvious whether this is actually safe or not.

In fact, thinking some more about that, I don't think there should be any need for this sort of logic in these handlers at all. The fixup_exception() machinery should already know enough about the exception that's happened and the extable entry to figure this out and not bother calling the handler at all.

Thanks,
Robin.




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