Messages in this thread | | | Date | Tue, 09 Nov 2021 06:57:47 -0800 | From | "Andy Lutomirski" <> | Subject | Re: XSAVE / RDPKRU on Intel 11th Gen Core CPUs |
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On Tue, Nov 9, 2021, at 5:43 AM, Brian Geffon wrote: > Hi Dave, > > On Tue, Nov 9, 2021 at 1:49 AM Dave Hansen <dave.hansen@intel.com> wrote: >> Well, gosh, it's making it back to the software init value. If you do: >> >> echo 0x15555554 > /sys/kernel/debug/x86/init_pkru >> >> do you end up with 0x15555554 as the value? > > What's interesting is that writing to init_pkru fails with -EINVAL for me, > and I've traced it down to get_xsave_addr() returning NULL on the following > check: > > /* > * This assumes the last 'xsave*' instruction to > * have requested that 'xfeature_nr' be saved. > * If it did not, we might be seeing and old value > * of the field in the buffer. > * > * This can happen because the last 'xsave' did not > * request that this feature be saved (unlikely) > * or because the "init optimization" caused it > * to not be saved. > */ > if (!(xsave->header.xfeatures & BIT_ULL(xfeature_nr))) > return NULL;
Here's an excerpt from an old email that I, perhaps unwisely, sent to Dave but not to a public list:
static inline void write_pkru(u32 pkru) { struct pkru_state *pk;
if (!boot_cpu_has(X86_FEATURE_OSPKE)) return;
pk = get_xsave_addr(¤t->thread.fpu.state.xsave, XFEATURE_PKRU);
/* * The PKRU value in xstate needs to be in sync with the value that is * written to the CPU. The FPU restore on return to userland would * otherwise load the previous value again. */ fpregs_lock(); if (pk) pk->pkru = pkru;
^^^ else we just write to the PKRU register but leave XINUSE[PKRU] clear on return to usermode? That seems... unwise.
__write_pkru(pkru); fpregs_unlock(); }
I bet you're hitting exactly this bug. The fix ended up being a whole series of patches, but the gist of it is that the write_pkru() slow path needs to set the xfeature bit in the xsave buffer and then do the write. It should be possible to make a little patch to do just this in a couple lines of code.
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