From 4a804c4f8356393d6b5eff7600f07615d7869c13 Mon Sep 17 00:00:00 2001 From: Kyle Huey Date: Tue, 15 Nov 2022 15:09:30 -0800 Subject: [PATCH] x86/fpu: Allow PKRU to be (once again) written by ptrace. Move KVM's PKRU handling code in fpu_copy_uabi_to_guest_fpstate() to copy_uabi_to_xstate() so that it is shared with other APIs that write the XSTATE such as PTRACE_SETREGSET with NT_X86_XSTATE. This restores the pre-5.14 behavior of ptrace. The regression can be seen by running gdb and executing `p $pkru`, `set $pkru = 42`, and `p $pkru`. On affected kernels (5.14+) the write to the PKRU register (which gdb performs through ptrace) is ignored. [ dhansen: removed stable@ tag for now. The ABI was broken for long enough that this is not urgent material. Let's let it stew in tip for a few weeks before it's submitted to stable because there are so many ABIs potentially affected. ] Fixes: e84ba47e313d ("x86/fpu: Hook up PKRU into ptrace()") Signed-off-by: Kyle Huey Signed-off-by: Dave Hansen Link: https://lore.kernel.org/all/20221115230932.7126-5-khuey%40kylehuey.com --- arch/x86/kernel/fpu/core.c | 13 +------------ arch/x86/kernel/fpu/xstate.c | 21 ++++++++++++++++++++- 2 files changed, 21 insertions(+), 13 deletions(-) diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c index 550157686323..46b935bc87c8 100644 --- a/arch/x86/kernel/fpu/core.c +++ b/arch/x86/kernel/fpu/core.c @@ -391,8 +391,6 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf, { struct fpstate *kstate = gfpu->fpstate; const union fpregs_state *ustate = buf; - struct pkru_state *xpkru; - int ret; if (!cpu_feature_enabled(X86_FEATURE_XSAVE)) { if (ustate->xsave.header.xfeatures & ~XFEATURE_MASK_FPSSE) @@ -406,16 +404,7 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf, if (ustate->xsave.header.xfeatures & ~xcr0) return -EINVAL; - ret = copy_uabi_from_kernel_to_xstate(kstate, ustate, vpkru); - if (ret) - return ret; - - /* Retrieve PKRU if not in init state */ - if (kstate->regs.xsave.header.xfeatures & XFEATURE_MASK_PKRU) { - xpkru = get_xsave_addr(&kstate->regs.xsave, XFEATURE_PKRU); - *vpkru = xpkru->pkru; - } - return 0; + return copy_uabi_from_kernel_to_xstate(kstate, ustate, vpkru); } EXPORT_SYMBOL_GPL(fpu_copy_uabi_to_guest_fpstate); #endif /* CONFIG_KVM */ diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c index d657c8b1fb08..a8cf604d8a25 100644 --- a/arch/x86/kernel/fpu/xstate.c +++ b/arch/x86/kernel/fpu/xstate.c @@ -1205,10 +1205,22 @@ static int copy_from_buffer(void *dst, unsigned int offset, unsigned int size, * @fpstate: The fpstate buffer to copy to * @kbuf: The UABI format buffer, if it comes from the kernel * @ubuf: The UABI format buffer, if it comes from userspace - * @pkru: unused + * @pkru: The location to write the PKRU value to * * Converts from the UABI format into the kernel internal hardware * dependent format. + * + * This function ultimately has three different callers with distinct PKRU + * behavior. + * 1. When called from sigreturn the PKRU register will be restored from + * @fpstate via an XRSTOR. Correctly copying the UABI format buffer to + * @fpstate is sufficient to cover this case, but the caller will also + * pass a pointer to the thread_struct's pkru field in @pkru and updating + * it is harmless. + * 2. When called from ptrace the PKRU register will be restored from the + * thread_struct's pkru field. A pointer to that is passed in @pkru. + * 3. When called from KVM the PKRU register will be restored from the vcpu's + * pkru field. A pointer to that is passed in @pkru. */ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf, const void __user *ubuf, u32 *pkru) @@ -1260,6 +1272,13 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf, } } + if (hdr.xfeatures & XFEATURE_MASK_PKRU) { + struct pkru_state *xpkru; + + xpkru = __raw_xsave_addr(xsave, XFEATURE_PKRU); + *pkru = xpkru->pkru; + } + /* * The state that came in from userspace was user-state only. * Mask all the user states out of 'xfeatures':