On 2/15/22 07:36, Brian Geffon wrote: > There are two issues with PKRU handling prior to 5.13. Are you sure both of these issues were introduced by 0cecca9d03c? I'm surprised that the get_xsave_addr() issue is not older. Should this be two patches? > The first is that when eagerly switching PKRU we check that current Don't forget to write in imperative mood. No "we's", please. https://www.kernel.org/doc/html/latest/process/maintainer-tip.html This goes for changelogs and comments too. > is not a kernel thread as kernel threads will never use PKRU. It's > possible that this_cpu_read_stable() on current_task (ie. > get_current()) is returning an old cached value. By forcing the read > with this_cpu_read() the correct task is used. Without this it's > possible when switching from a kernel thread to a userspace thread > that we'll still observe the PF_KTHREAD flag and never restore the > PKRU. And as a result this issue only occurs when switching from a > kernel thread to a userspace thread, switching from a non kernel > thread works perfectly fine because all we consider in that situation > is the flags from some other non kernel task and the next fpu is > passed in to switch_fpu_finish(). It makes *sense* that there would be a place in the context switch code where 'current' is wonky, but I never realized this. This seems really fragile, but *also* trivially detectable. Is the PKRU code really the only code to use 'current' in a buggy way like this? > The second issue is when using write_pkru() we only write to the > xstate when the feature bit is set because get_xsave_addr() returns > NULL when the feature bit is not set. This is problematic as the CPU > is free to clear the feature bit when it observes the xstate in the > init state, this behavior seems to be documented a few places throughout > the kernel. If the bit was cleared then in write_pkru() we would happily > write to PKRU without ever updating the xstate, and the FPU restore on > return to userspace would load the old value agian. ^ again It's probably worth noting that the AMD init tracker is a lot more aggressive than Intel's. On Intel, I think XRSTOR is the only way to get back to the init state. You're obviously hitting this on AMD. It's also *very* unlikely that PKRU gets back to a value of 0. I think we added a selftest for this case in later kernels. That helps explain why this bug hung around for so long. > diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h > index 03b3de491b5e..540bda5bdd28 100644 > --- a/arch/x86/include/asm/fpu/internal.h > +++ b/arch/x86/include/asm/fpu/internal.h > @@ -598,7 +598,7 @@ static inline void switch_fpu_finish(struct fpu *new_fpu) > * PKRU state is switched eagerly because it needs to be valid before we > * return to userland e.g. for a copy_to_user() operation. > */ > - if (!(current->flags & PF_KTHREAD)) { > + if (!(this_cpu_read(current_task)->flags & PF_KTHREAD)) { This really deserves a specific comment. > /* > * If the PKRU bit in xsave.header.xfeatures is not set, > * then the PKRU component was in init state, which means > diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h > index 9e71bf86d8d0..aa381b530de0 100644 > --- a/arch/x86/include/asm/pgtable.h > +++ b/arch/x86/include/asm/pgtable.h > @@ -140,16 +140,22 @@ static inline void write_pkru(u32 pkru) > 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; > + /* > + * The CPU is free to clear the feature bit when the xstate is in the > + * init state. For this reason, we need to make sure the feature bit is > + * reset when we're explicitly writing to pkru. If we did not then we > + * would write to pkru and it would not be saved on a context switch. > + */ > + current->thread.fpu.state.xsave.header.xfeatures |= XFEATURE_MASK_PKRU; I don't think we need to describe how the init optimization works again. I'm also not sure it's worth mentioning context switches here. It's a wider problem than that. Maybe: /* * All fpregs will be XRSTOR'd from this buffer before returning * to userspace. Ensure that XRSTOR does not init PKRU and that * get_xsave_addr() will work. */ > + pk = get_xsave_addr(¤t->thread.fpu.state.xsave, XFEATURE_PKRU); > + BUG_ON(!pk); A BUG_ON() a line before a NULL pointer dereference doesn't tend to do much good. > + pk->pkru = pkru; > __write_pkru(pkru); > fpregs_unlock(); > }
