Hi Jessica,
On 16/10/2024 00:04, Jessica Clarke wrote:
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?
Not sure why fortify complains here, and I have just seen that I forgot
to cc Kees (done now).
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.
I'm not sure to follow: you propose to use unsafe_memcpy() instead of
disabling fortify entirely, so we would not get any warning in case of
failure anyway right? Or do you propose to modify the fortify code to
somehow print a warning? If the latter, it's hard this soon in the boot
process (where the mmu is disabled) to make sure that the printing
warning path does not try to access any virtual address (which is why
the boot failed in the first place) but maybe Kees has an idea.
And I believe that enabling fortify and using the unsafe_*() variants is
error-prone since we'd have to make sure that all the "fortified"
functions used in that code use the unsafe_*() variants.
So to me, it's way easier in terms of maintenance to just disabling fortify.
Thanks,
Alex
Jess
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