On Mon, Nov 15, 2021 at 04:30:20PM +0000, Jane Malalane wrote:
commit 415de44076640483648d6c0f6d645a9ee61328ad upstream.
Currently, Linux probes for X86_BUG_NULL_SEL unconditionally which makes it unsafe to migrate in a virtualised environment as the properties across the migration pool might differ.
To be specific, the case which goes wrong is:
- Zen1 (or earlier) and Zen2 (or later) in a migration pool
- Linux boots on Zen2, probes and finds the absence of X86_BUG_NULL_SEL
- Linux is then migrated to Zen1
Linux is now running on a X86_BUG_NULL_SEL-impacted CPU while believing that the bug is fixed.
The only way to address the problem is to fully trust the "no longer affected" CPUID bit when virtualised, because in the above case it would be clear deliberately to indicate the fact "you might migrate to somewhere which has this behaviour".
Zen3 adds the NullSelectorClearsBase CPUID bit to indicate that loading a NULL segment selector zeroes the base and limit fields, as well as just attributes. Zen2 also has this behaviour but doesn't have the NSCB bit.
[ bp: Minor touchups. ]
Signed-off-by: Jane Malalane jane.malalane@citrix.com Signed-off-by: Borislav Petkov bp@suse.de CC: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20211021104744.24126-1-jane.malalane@citrix.com
Backport to 4.19. Drop Hygon modifications.
Now queued up, thanks.
greg k-h