x86/vdso: Prevent segfaults due to hoisted vclock reads
commit ff17bbe0 upstream. GCC 5.5.0 sometimes cleverly hoists reads of the pvclock and/or hvclock pages before the vclock mode checks. This creates a path through vclock_gettime() in which no vclock is enabled at all (due to disabled TSC on old CPUs, for example) but the pvclock or hvclock page nevertheless read. This will segfault on bare metal. This fixes commit 459e3a21 ("gcc-9: properly declare the {pv,hv}clock_page storage") in the sense that, before that commit, GCC didn't seem to generate the offending code. There was nothing wrong with that commit per se, and -stable maintainers should backport this to all supported kernels regardless of whether the offending commit was present, since the same crash could just as easily be triggered by the phase of the moon. On GCC 9.1.1, this doesn't seem to affect the generated code at all, so I'm not too concerned about performance regressions from this fix. Cc: stable@vger.kernel.org Cc: x86@kernel.org Cc: Borislav Petkov <bp@alien8.de> Reported-by: Duncan Roe <duncan_roe@optusnet.com.au> Signed-off-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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