[tip:x86/urgent] x86/mm/pkeys: Fix compact mode by removing protection keys' XSAVE buffer manipulation
From: tip-bot for Dave Hansen
Date: Wed Aug 10 2016 - 16:37:38 EST
Commit-ID: b79daf85899215d5ede3641806db2e2a77b776b4
Gitweb: http://git.kernel.org/tip/b79daf85899215d5ede3641806db2e2a77b776b4
Author: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
AuthorDate: Wed, 27 Jul 2016 16:20:40 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 10 Aug 2016 16:12:26 +0200
x86/mm/pkeys: Fix compact mode by removing protection keys' XSAVE buffer manipulation
The Memory Protection Keys "rights register" (PKRU) is
XSAVE-managed, and is saved/restored along with the FPU state.
When kernel code accesses FPU regsisters, it does a delicate
dance with preempt. Otherwise, the context switching code can
get confused as to whether the most up-to-date state is in the
registers themselves or in the XSAVE buffer.
But, PKRU is not a normal FPU register. Using it does not
generate the normal device-not-available (#NM) exceptions which
means we can not manage it lazily, and the kernel completley
disallows using lazy mode when it is enabled.
The dance with preempt *only* occurs when managing the FPU
lazily. Since we never manage PKRU lazily, we do not have to do
the dance with preempt; we can access it directly. Doing it
this way saves a ton of complicated code (and is faster too).
Further, the XSAVES reenabling failed to patch a bit of code
in fpu__xfeature_set_state() the checked for compacted buffers.
That check caused fpu__xfeature_set_state() to silently refuse to
work when the kernel is using compacted XSAVE buffers. This
broke execute-only and future pkey_mprotect() support when using
compact XSAVE buffers.
But, removing fpu__xfeature_set_state() gets rid of this issue,
in addition to the nice cleanup and speedup.
This fixes the same thing as a fix that Sai posted:
https://lkml.org/lkml/2016/7/25/637
The fix that he posted is a much more obviously correct, but I
think we should just do this instead.
Reported-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Dave Hansen <dave@xxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>
Cc: Ravi Shankar <ravi.v.shankar@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Yu-Cheng Yu <yu-cheng.yu@xxxxxxxxx>
Link: http://lkml.kernel.org/r/20160727232040.7D060DAD@xxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/kernel/fpu/xstate.c | 138 ++++++-------------------------------------
1 file changed, 17 insertions(+), 121 deletions(-)
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 680049a..01567aa 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -866,105 +866,17 @@ const void *get_xsave_field_ptr(int xsave_state)
return get_xsave_addr(&fpu->state.xsave, xsave_state);
}
-
-/*
- * Set xfeatures (aka XSTATE_BV) bit for a feature that we want
- * to take out of its "init state". This will ensure that an
- * XRSTOR actually restores the state.
- */
-static void fpu__xfeature_set_non_init(struct xregs_state *xsave,
- int xstate_feature_mask)
-{
- xsave->header.xfeatures |= xstate_feature_mask;
-}
-
-/*
- * This function is safe to call whether the FPU is in use or not.
- *
- * Note that this only works on the current task.
- *
- * Inputs:
- * @xsave_state: state which is defined in xsave.h (e.g. XFEATURE_MASK_FP,
- * XFEATURE_MASK_SSE, etc...)
- * @xsave_state_ptr: a pointer to a copy of the state that you would
- * like written in to the current task's FPU xsave state. This pointer
- * must not be located in the current tasks's xsave area.
- * Output:
- * address of the state in the xsave area or NULL if the state
- * is not present or is in its 'init state'.
- */
-static void fpu__xfeature_set_state(int xstate_feature_mask,
- void *xstate_feature_src, size_t len)
-{
- struct xregs_state *xsave = ¤t->thread.fpu.state.xsave;
- struct fpu *fpu = ¤t->thread.fpu;
- void *dst;
-
- if (!boot_cpu_has(X86_FEATURE_XSAVE)) {
- WARN_ONCE(1, "%s() attempted with no xsave support", __func__);
- return;
- }
-
- /*
- * Tell the FPU code that we need the FPU state to be in
- * 'fpu' (not in the registers), and that we need it to
- * be stable while we write to it.
- */
- fpu__current_fpstate_write_begin();
-
- /*
- * This method *WILL* *NOT* work for compact-format
- * buffers. If the 'xstate_feature_mask' is unset in
- * xcomp_bv then we may need to move other feature state
- * "up" in the buffer.
- */
- if (xsave->header.xcomp_bv & xstate_feature_mask) {
- WARN_ON_ONCE(1);
- goto out;
- }
-
- /* find the location in the xsave buffer of the desired state */
- dst = __raw_xsave_addr(&fpu->state.xsave, xstate_feature_mask);
-
- /*
- * Make sure that the pointer being passed in did not
- * come from the xsave buffer itself.
- */
- WARN_ONCE(xstate_feature_src == dst, "set from xsave buffer itself");
-
- /* put the caller-provided data in the location */
- memcpy(dst, xstate_feature_src, len);
-
- /*
- * Mark the xfeature so that the CPU knows there is state
- * in the buffer now.
- */
- fpu__xfeature_set_non_init(xsave, xstate_feature_mask);
-out:
- /*
- * We are done writing to the 'fpu'. Reenable preeption
- * and (possibly) move the fpstate back in to the fpregs.
- */
- fpu__current_fpstate_write_end();
-}
-
#define NR_VALID_PKRU_BITS (CONFIG_NR_PROTECTION_KEYS * 2)
#define PKRU_VALID_MASK (NR_VALID_PKRU_BITS - 1)
/*
- * This will go out and modify the XSAVE buffer so that PKRU is
- * set to a particular state for access to 'pkey'.
- *
- * PKRU state does affect kernel access to user memory. We do
- * not modfiy PKRU *itself* here, only the XSAVE state that will
- * be restored in to PKRU when we return back to userspace.
+ * This will go out and modify PKRU register to set the access
+ * rights for @pkey to @init_val.
*/
int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
unsigned long init_val)
{
- struct xregs_state *xsave = &tsk->thread.fpu.state.xsave;
- struct pkru_state *old_pkru_state;
- struct pkru_state new_pkru_state;
+ u32 old_pkru;
int pkey_shift = (pkey * PKRU_BITS_PER_PKEY);
u32 new_pkru_bits = 0;
@@ -974,6 +886,15 @@ int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
*/
if (!boot_cpu_has(X86_FEATURE_OSPKE))
return -EINVAL;
+ /*
+ * For most XSAVE components, this would be an arduous task:
+ * brining fpstate up to date with fpregs, updating fpstate,
+ * then re-populating fpregs. But, for components that are
+ * never lazily managed, we can just access the fpregs
+ * directly. PKRU is never managed lazily, so we can just
+ * manipulate it directly. Make sure it stays that way.
+ */
+ WARN_ON_ONCE(!use_eager_fpu());
/* Set the bits we need in PKRU: */
if (init_val & PKEY_DISABLE_ACCESS)
@@ -984,37 +905,12 @@ int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
/* Shift the bits in to the correct place in PKRU for pkey: */
new_pkru_bits <<= pkey_shift;
- /* Locate old copy of the state in the xsave buffer: */
- old_pkru_state = get_xsave_addr(xsave, XFEATURE_MASK_PKRU);
-
- /*
- * When state is not in the buffer, it is in the init
- * state, set it manually. Otherwise, copy out the old
- * state.
- */
- if (!old_pkru_state)
- new_pkru_state.pkru = 0;
- else
- new_pkru_state.pkru = old_pkru_state->pkru;
-
- /* Mask off any old bits in place: */
- new_pkru_state.pkru &= ~((PKRU_AD_BIT|PKRU_WD_BIT) << pkey_shift);
-
- /* Set the newly-requested bits: */
- new_pkru_state.pkru |= new_pkru_bits;
-
- /*
- * We could theoretically live without zeroing pkru.pad.
- * The current XSAVE feature state definition says that
- * only bytes 0->3 are used. But we do not want to
- * chance leaking kernel stack out to userspace in case a
- * memcpy() of the whole xsave buffer was done.
- *
- * They're in the same cacheline anyway.
- */
- new_pkru_state.pad = 0;
+ /* Get old PKRU and mask off any old bits in place: */
+ old_pkru = read_pkru();
+ old_pkru &= ~((PKRU_AD_BIT|PKRU_WD_BIT) << pkey_shift);
- fpu__xfeature_set_state(XFEATURE_MASK_PKRU, &new_pkru_state, sizeof(new_pkru_state));
+ /* Write old part along with new part: */
+ write_pkru(old_pkru | new_pkru_bits);
return 0;
}