Re: [PATCH] arm: explicitly place .fixup in .text

[Nicolas Pitre]

On Fri, 22 Nov 2019, Nick Desaulniers wrote:

> From: Kees Cook <keescook@xxxxxxxxxxxx>
> 
> There's an implicit dependency on the section ordering of the orphaned
> section .fixup that can break arm_copy_from_user if the linker places
> the .fixup section before the .text section. Since .fixup is not
> explicitly placed in the existing ARM linker scripts, the linker is free
> to order it anywhere with respect to the rest of the sections.
> 
> Multiple users from different distros (Raspbian, CrOS) reported kernel
> panics executing seccomp() syscall with Linux kernels linked with LLD.
> 
> Documentation/x86/exception-tables.rst alludes to the ordering
> dependency. The relevant quote:
> 
> ```
> NOTE:
> Due to the way that the exception table is built and needs to be ordered,
> only use exceptions for code in the .text section.  Any other section
> will cause the exception table to not be sorted correctly, and the
> exceptions will fail.
> 
> Things changed when 64-bit support was added to x86 Linux. Rather than
> double the size of the exception table by expanding the two entries
> from 32-bits to 64 bits, a clever trick was used to store addresses
> as relative offsets from the table itself. The assembly code changed
> from::
> 
>     .long 1b,3b
>   to:
>           .long (from) - .
>           .long (to) - .
> 
> and the C-code that uses these values converts back to absolute addresses
> like this::
> 
>         ex_insn_addr(const struct exception_table_entry *x)
>         {
>                 return (unsigned long)&x->insn + x->insn;
>         }
> ```
> 
> Since the addresses stored in the __ex_table are RELATIVE offsets and
> not ABSOLUTE addresses, ordering the fixup anywhere that's not
> immediately preceding .text causes the relative offset of the faulting
> instruction to be wrong, causing the wrong (or no) address of the fixup
> handler to looked up in __ex_table.

This explanation makes no sense.

The above is valid only when ARCH_HAS_RELATIVE_EXTABLE is defined. On 
ARM32 it is not, nor would it make sense to be.


Nicolas