Re: [PATCH -fixes] riscv: Do not use fortify in early code

[Jessica Clarke]

On 9 Oct 2024, at 08:27, Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
> 
> Early code designates the code executed when the MMU is not yet enabled,
> and this comes with some limitations (see
> Documentation/arch/riscv/boot.rst, section "Pre-MMU execution").
> 
> FORTIFY_SOURCE must be disabled then since it can trigger kernel panics
> as reported in [1].
> 
> Reported-by: Jason Montleon <jmontleo@xxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-riscv/CAJD_bPJes4QhmXY5f63GHV9B9HFkSCoaZjk-qCT2NGS7Q9HODg@xxxxxxxxxxxxxx/ [1]
> Fixes: a35707c3d850 ("riscv: add memory-type errata for T-Head")
> Fixes: 26e7aacb83df ("riscv: Allow to downgrade paging mode from the command line")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>

Is the problem in [1] not just that the early boot path uses memcpy on
the result of ALT_OLD_PTR, which is a wildly out-of-bounds pointer from
the compiler’s perspective? If so, it would seem better to use
unsafe_memcpy for that one call site rather than use the big
__NO_FORTIFY hammer, surely?

Presumably the non-early path is just as bad to the compiler, but works
because patch_text_nosync isn’t instrumented, so that would just align
the two.

Getting the implementation to not be silent on failure during early
boot would also be a good idea, but it’s surely better to have
FORTIFY_SOURCE enabled with no output for positives than disable the
checking in the first place and risk uncaught corruption.

Jess