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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@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yu-Cheng Yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/20160727232040.7D060DAD@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Dave Hansen 2016-07-27 16:20:40 -07:00 committed by Ingo Molnar
parent 5e44258d16
commit b79daf8589
1 changed files with 17 additions and 121 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

138
arch/x86/kernel/fpu/xstate.c
View File

@ -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 = &current->thread.fpu.state.xsave;
struct fpu *fpu = &current->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);
/* 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);
/*
* 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;
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;
}