RE: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for init_fpstate

[Yao, Yuan]

>-----Original Message-----
>From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>Sent: Wednesday, October 12, 2022 06:24
>To: linux-kernel@xxxxxxxxxxxxxxx
>Cc: Bae, Chang Seok <chang.seok.bae@xxxxxxxxx>; x86@xxxxxxxxxx; Yao, Yuan <yuan.yao@xxxxxxxxx>; Hansen, Dave
><dave.hansen@xxxxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>Subject: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for init_fpstate
>
>From: Yuan Yao <yuan.yao@xxxxxxxxx>
>
>This was found a couple of months ago in a big old AMX
>backport.  But, it looks to be a problem in mainline too.
>Please let me know if this looks OK.  I'd also especially
>appreciate some testing from folks that have AMX hardware
>handy.
>
>Builds and survives a quick boot test on non-AMX hardware.
>
>--
>
>== Background ==
>
>'init_fpstate' is a sort of template for all of the fpstates
>that come after it.  It is copied when new processes are
>execve()'d or XRSTOR'd to get fpregs into a known state.
>
>That means that it represents the *starting* state for a
>process's fpstate which includes only the 'default' features.
>Dynamic features can, of course, be acquired later, but
>processes start with only default_features.
>
>During boot the kernel decides whether all fpstates will be
>kept in the compacted or uncompacted format.  This choice is
>communicated to the hardware via the XCOMP_BV field in all
>XSAVE buffers, including 'init_fpstate'.
>
>== Problem ==
>
>But, the existing XCOMP_BV calculation is incorrect.  It uses
>the set of 'max_features', not the default features.
>
>As a result, when XRSTOR operates on buffers derived from
>'init_fpstate', it may attempt to "tickle" dynamic features which
>are at offsets for which there is no space allocated in the
>fpstate.
>
>== Scope ==
>
>This normally results in a relatively harmless out-of-bounds
>memory read.  It's harmless because it never gets consumed.  But,
>if the fpstate is next to some unmapped memory, this "tickle" can
>cause a page fault and an oops.
>
>This only causes issues on systems when dynamic features are
>available and when an XSAVE buffer is next to uninitialized
>memory.  In other words, it only affects recent Intel server
>CPUs, and in relatively few memory locations.
>
>Those two things are why it took relatively long to catch this.
>
>== Solution ==
>
>Use 'default_features' to establish the init_fpstate
>xcomp_bv value.  Reset individual fpstate xcomp_bv values
>when the rest of the fpstate is reset.
>
>[ dhansen: add reset code from tglx, rewrites
>	   commit message and comments ]

Thanks to Dave's help on this! The new comment message is more
clear and easier to understand then before.

>
>Fixes: 1c253ff2287f ("x86/fpu: Move xstate feature masks to fpu_*_cfg")
>Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
>Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>Signed-off-by: Yuan Yao <yuan.yao@xxxxxxxxx>
>Cc: stable@xxxxxxxxxxxxxxx
>---
> arch/x86/kernel/fpu/core.c   | 3 +++
> arch/x86/kernel/fpu/xstate.c | 7 ++++++-
> 2 files changed, 9 insertions(+), 1 deletion(-)
>
>diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
>index 3b28c5b25e12..4d64de34da12 100644
>--- a/arch/x86/kernel/fpu/core.c
>+++ b/arch/x86/kernel/fpu/core.c
>@@ -526,6 +526,9 @@ static void __fpstate_reset(struct fpstate *fpstate, u64 xfd)
> 	fpstate->xfeatures	= fpu_kernel_cfg.default_features;
> 	fpstate->user_xfeatures	= fpu_user_cfg.default_features;
> 	fpstate->xfd		= xfd;
>+
>+	/* Ensure that xcomp_bv matches ->xfeatures */
>+	xstate_init_xcomp_bv(&fpstate->regs.xsave, fpstate->xfeatures);
> }
>
> void fpstate_reset(struct fpu *fpu)
>diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
>index c8340156bfd2..f9f45610c72f 100644
>--- a/arch/x86/kernel/fpu/xstate.c
>+++ b/arch/x86/kernel/fpu/xstate.c
>@@ -360,7 +360,12 @@ static void __init setup_init_fpu_buf(void)
>
> 	print_xstate_features();
>
>-	xstate_init_xcomp_bv(&init_fpstate.regs.xsave, fpu_kernel_cfg.max_features);
>+	/*
>+	 * 'init_fpstate' is sized for the default feature
>+	 * set without any of the dynamic features.
>+	 */
>+	xstate_init_xcomp_bv(&init_fpstate.regs.xsave,
>+			     fpu_kernel_cfg.default_features);
>
> 	/*
> 	 * Init all the features state with header.xfeatures being 0x0
>--
>2.34.1