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Subject: Re: [PATCH 4/5] x86/pkeys: replace PKRU modification infrastructure
To:     Dave Hansen <dave.hansen@linux.intel.com>, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, x86@kernel.org, luto@kernel.org, shuah@kernel.org,
        babu.moger@amd.com, linuxram@us.ibm.com, bauerman@linux.ibm.com,
        bigeasy@linutronix.de
References: <20210527235109.B2A9F45F@viggo.jf.intel.com>
 <20210527235118.88C9831B@viggo.jf.intel.com>
From:   Dave Kleikamp <dave.kleikamp@oracle.com>
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Date:   Fri, 28 May 2021 12:00:36 -0500
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Noticed a typo in a comment, but I haven't reviewed these thoroughly.

Shaggy

On 5/27/21 6:51 PM, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> There are two points in the kernel which write to PKRU in a buggy way:
> 
>   * In switch_fpu_finish(), where having xfeatures[PKRU]=0 will result
>     in PKRU being assigned 'init_pkru_value' instead of 0x0.
>   * In write_pkru(), xfeatures[PKRU]=0 will result in PKRU having the
>     correct value, but the XSAVE buffer will remain stale because
>     xfeatures is not updated.
> 
> Both of them screw up the fact that get_xsave_addr() will return NULL
> for PKRU when it is in the XSAVE "init state".  This went unnoticed
> until now because on Intel hardware XINUSE[PKRU] is never 0 because
> of the kernel policy around 'init_pkru_value'.  AMD hardware, on the
> other hand, can set XINUSE[PKRU]=0 via a normal WRPKRU(0).  The
> handy selftests somewhat accidentally produced a case[2] where
> WRPKRU(0) occurs.
> 
> get_xsave_addr() is a horrible interface[1], especially when used for
> writing state.  It is too easy for callers to be tricked into thinking:
>   1. On a NULL return that they have no work to do
>   2. On a valid pointer return that they *can* safely write state
>      without doing more work like setting an xfeatures bit.
> 
> Wrap get_xsave_addr() with some additional infrastructure.  Ensure
> that callers must declare their intent to read or write to the state.
> Inject the init state into both reads *and* writes.  This ensures
> that writers never have to deal with detritus from previous state.
> 
> The new common xstate infrastructure:
> 
> 	xstatebuf_get_write_ptr()
> and
> 	xfeature_init_space()
> 
> should be quite usable for other xfeatures with trivial updates to
> xfeature_init_space().  My hope is that we can move away from
> all use of get_xsave_addr(), replacing it with things like
> xstate_read_pkru().
> 
> The new BUG_ON()s are not great.  But, they do represent a severe
> violation of expectations and XSAVE state can be security-sensitive
> and these represent a truly dazed-and-confused situation.
> 
> 1. I know, I wrote it.  I'm really sorry.
> 2. https://lore.kernel.org/linux-kselftest/b2e0324a-9125-bb34-9e76-81817df27c48@amd.com/
> 
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Fixes: 0d714dba1626 ("x86/fpu: Update xstate's PKRU value on write_pkru()")
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: x86@kernel.org
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Babu Moger <babu.moger@amd.com>
> Cc: Dave Kleikamp <dave.kleikamp@oracle.com>
> Cc: Ram Pai <linuxram@us.ibm.com>
> Cc: Thiago Jung Bauermann <bauerman@linux.ibm.com>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> 
>   b/arch/x86/include/asm/fpu/internal.h |    8 --
>   b/arch/x86/include/asm/fpu/xstate.h   |  111 +++++++++++++++++++++++++++++++---
>   b/arch/x86/include/asm/processor.h    |    7 ++
>   b/arch/x86/kernel/cpu/common.c        |    6 -
>   b/arch/x86/mm/pkeys.c                 |    6 -
>   5 files changed, 115 insertions(+), 23 deletions(-)
> 
> diff -puN arch/x86/include/asm/fpu/internal.h~write_pkru arch/x86/include/asm/fpu/internal.h
> --- a/arch/x86/include/asm/fpu/internal.h~write_pkru	2021-05-27 16:40:26.903705463 -0700
> +++ b/arch/x86/include/asm/fpu/internal.h	2021-05-27 16:40:26.919705463 -0700
> @@ -564,7 +564,6 @@ static inline void switch_fpu_prepare(st
>   static inline void switch_fpu_finish(struct fpu *new_fpu)
>   {
>   	u32 pkru_val = init_pkru_value;
> -	struct pkru_state *pk;
>   
>   	if (!static_cpu_has(X86_FEATURE_FPU))
>   		return;
> @@ -578,11 +577,8 @@ static inline void switch_fpu_finish(str
>   	 * 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->mm) {
> -		pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
> -		if (pk)
> -			pkru_val = pk->pkru;
> -	}
> +	if (current->mm)
> +		pkru_val = xstate_read_pkru(&new_fpu->state.xsave);
>   	__write_pkru(pkru_val);
>   
>   	/*
> diff -puN arch/x86/include/asm/fpu/xstate.h~write_pkru arch/x86/include/asm/fpu/xstate.h
> --- a/arch/x86/include/asm/fpu/xstate.h~write_pkru	2021-05-27 16:40:26.906705463 -0700
> +++ b/arch/x86/include/asm/fpu/xstate.h	2021-05-27 16:40:26.919705463 -0700
> @@ -124,27 +124,124 @@ static inline u32 read_pkru(void)
>   	return 0;
>   }
>   
> +static inline void xfeature_mark_non_init(struct xregs_state *xstate,
> +					  int xfeature_nr)
> +{
> +	/*
> +	 * Caller will place data in the @xstate buffer.
> +	 * Mark the xfeature as non-init:
> +	 */
> +	xstate->header.xfeatures |= BIT_ULL(xfeature_nr);
> +}
> +
> +
> +/* Set the contents of @xfeature_nr to the hardware init state */
> +static inline void xfeature_init_space(struct xregs_state *xstate,
> +					     int xfeature_nr)
> +{
> +	void *state = get_xsave_addr(xstate, xfeature_nr);
> +
> +	switch (xfeature_nr) {
> +	case XFEATURE_PKRU:
> +		/* zero the whole state, including reserved bits */
> +		memset(state, 0, sizeof(struct pkru_state));
> +		break;
> +	default:
> +		BUG();
> +	}
> +}
> +
> +/*
> + * Called when it is necessary to write to an XSAVE
> + * component feature.  Guarantees that a future
> + * XRSTOR of the 'xstate' buffer will not consider
> + * @xfeature_nr to be in its init state.
> + *
> + * The returned buffer may contain old state.  The
> + * caller must be prepared to fill the entire buffer.
> + *
> + * Caller must first ensure that @xfeature_nr is
> + * enabled and present in the @xstate buffer.
> + */
> +static inline void *xstatebuf_get_write_ptr(struct xregs_state *xstate,
> +					    int xfeature_nr)
> +{
> +	bool feature_was_init = xstate->header.xfeatures & BIT_ULL(xfeature_nr);
> +
> +	/*
> +	 * xcomp_bv represents whether 'xstate' has space for
> +	 * features.  If not, something is horribly wrong and
> +	 * a write would corrupt memory.  Perhaps xfeature_nr
> +	 * was not enabled.
> +	 */
> +	BUG_ON(!(xstate->header.xcomp_bv & BIT_ULL(xfeature_nr)));
> +
> +	/*
> +	 * Ensure a sane xfeature_nr, including avoiding
> +	 * confusion with XCOMP_BV_COMPACTED_FORMAT.
> +	 */
> +	BUG_ON(xfeature_nr >= XFEATURE_MAX);
> +
> +	/* Prepare xstate for a write to the xfeature: */
> +	xfeature_mark_non_init(xstate, xfeature_nr);
> +
> +	/*
> +	 * If xfeature_nr was in the init state, update the buffer
> +	 * to match the state. Ensures that callers can safely
> +	 * write only a part of the state, they are not forced to
> +	 * write it in its entirety.
> +	 */
> +	if (feature_was_init)
> +		xfeature_init_space(xstate, xfeature_nr);
> +
> +	return get_xsave_addr(xstate, xfeature_nr);
> +}
> +
> +/* Caller must ensure X86_FEATURE_OSPKE is enabled. */
> +static inline void xstate_write_pkru(struct xregs_state *xstate, u32 pkru)
> +{
> +	struct pkru_state *pk;
> +
> +	pk = xstatebuf_get_write_ptr(xstate, XFEATURE_PKRU);
> +	pk->pkru = pkru;
> +}
> +
> +/*
> + * What PKRU value is represented in the 'xstate'?  Note,
> + * this returns the *architecturally* represented value,
> + * not the literal in-memory value.  They may be different.
> + */
> +static inline u32 xstate_read_pkru(struct xregs_state *xstate)
> +{
> +	struct pkru_state *pk;
> +
> +	pk = get_xsave_addr(xstate, XFEATURE_PKRU);
> +	/*
> +	 * If present, pull PKRU out of the XSAVE buffer.
> +	 * Otherwise, use the hardware init value.
> +	 */
> +	if (pk)
> +		return pk->pkru;
> +	else
> +		return PKRU_HW_INIT_VALUE;
> +}
> +
>   /*
>    * Update all of the PKRU state for the current task:
>    * PKRU register and PKRU xstate.
>    */
>   static inline void current_write_pkru(u32 pkru)
>   {
> -	struct pkru_state *pk;
> -
>   	if (!boot_cpu_has(X86_FEATURE_OSPKE))
>   		return;
>   
> -	pk = get_xsave_addr(&current->thread.fpu.state.xsave, XFEATURE_PKRU);
> -
> +	fpregs_lock();
>   	/*
>   	 * 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;
> +	xstate_write_pkru(&current->thread.fpu.state.xsave, pkru);
>   	__write_pkru(pkru);
>   	fpregs_unlock();
>   }
> diff -puN arch/x86/include/asm/processor.h~write_pkru arch/x86/include/asm/processor.h
> --- a/arch/x86/include/asm/processor.h~write_pkru	2021-05-27 16:40:26.908705463 -0700
> +++ b/arch/x86/include/asm/processor.h	2021-05-27 16:40:26.921705463 -0700
> @@ -854,4 +854,11 @@ enum mds_mitigations {
>   	MDS_MITIGATION_VMWERV,
>   };
>   
> +/*
> + * The XSAVE architecture defines an "init state" for
> + * PKRU.  PKRU is set to this value by XRSTOR when it
> + * tries to restore PKRU but has on value in the buffer.

... but has *no* value ...

> + */
> +#define PKRU_HW_INIT_VALUE	0x0
> +
>   #endif /* _ASM_X86_PROCESSOR_H */
> diff -puN arch/x86/kernel/cpu/common.c~write_pkru arch/x86/kernel/cpu/common.c
> --- a/arch/x86/kernel/cpu/common.c~write_pkru	2021-05-27 16:40:26.912705463 -0700
> +++ b/arch/x86/kernel/cpu/common.c	2021-05-27 16:40:26.924705463 -0700
> @@ -466,8 +466,6 @@ static bool pku_disabled;
>   
>   static __always_inline void setup_pku(struct cpuinfo_x86 *c)
>   {
> -	struct pkru_state *pk;
> -
>   	/* check the boot processor, plus compile options for PKU: */
>   	if (!cpu_feature_enabled(X86_FEATURE_PKU))
>   		return;
> @@ -478,9 +476,7 @@ static __always_inline void setup_pku(st
>   		return;
>   
>   	cr4_set_bits(X86_CR4_PKE);
> -	pk = get_xsave_addr(&init_fpstate.xsave, XFEATURE_PKRU);
> -	if (pk)
> -		pk->pkru = init_pkru_value;
> +	xstate_write_pkru(&current->thread.fpu.state.xsave, init_pkru_value);
>   	/*
>   	 * Seting X86_CR4_PKE will cause the X86_FEATURE_OSPKE
>   	 * cpuid bit to be set.  We need to ensure that we
> diff -puN arch/x86/mm/pkeys.c~write_pkru arch/x86/mm/pkeys.c
> --- a/arch/x86/mm/pkeys.c~write_pkru	2021-05-27 16:40:26.914705463 -0700
> +++ b/arch/x86/mm/pkeys.c	2021-05-27 16:40:26.926705463 -0700
> @@ -155,7 +155,6 @@ static ssize_t init_pkru_read_file(struc
>   static ssize_t init_pkru_write_file(struct file *file,
>   		 const char __user *user_buf, size_t count, loff_t *ppos)
>   {
> -	struct pkru_state *pk;
>   	char buf[32];
>   	ssize_t len;
>   	u32 new_init_pkru;
> @@ -178,10 +177,7 @@ static ssize_t init_pkru_write_file(stru
>   		return -EINVAL;
>   
>   	WRITE_ONCE(init_pkru_value, new_init_pkru);
> -	pk = get_xsave_addr(&init_fpstate.xsave, XFEATURE_PKRU);
> -	if (!pk)
> -		return -EINVAL;
> -	pk->pkru = new_init_pkru;
> +	xstate_write_pkru(&init_fpstate.xsave, new_init_pkru);
>   	return count;
>   }
>   
> _
>