This is a note to let you know that I've just added the patch titled
x86/pkru: Write hardware init value to PKRU when xstate is init
to the 5.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-pkru-write-hardware-init-value-to-pkru-when-xstate-is-init.patch
and it can be found in the queue-5.12 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 510b80a6a0f1a0d114c6e33bcea64747d127973c Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date: Tue, 8 Jun 2021 16:36:21 +0200
Subject: x86/pkru: Write hardware init value to PKRU when xstate is init
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
commit 510b80a6a0f1a0d114c6e33bcea64747d127973c upstream.
When user space brings PKRU into init state, then the kernel handling is
broken:
T1 user space
xsave(state)
state.header.xfeatures &= ~XFEATURE_MASK_PKRU;
xrstor(state)
T1 -> kernel
schedule()
XSAVE(S) -> T1->xsave.header.xfeatures[PKRU] == 0
T1->flags |= TIF_NEED_FPU_LOAD;
wrpkru();
schedule()
...
pk = get_xsave_addr(&T1->fpu->state.xsave, XFEATURE_PKRU);
if (pk)
wrpkru(pk->pkru);
else
wrpkru(DEFAULT_PKRU);
Because the xfeatures bit is 0 and therefore the value in the xsave
storage is not valid, get_xsave_addr() returns NULL and switch_to()
writes the default PKRU. -> FAIL #1!
So that wrecks any copy_to/from_user() on the way back to user space
which hits memory which is protected by the default PKRU value.
Assumed that this does not fail (pure luck) then T1 goes back to user
space and because TIF_NEED_FPU_LOAD is set it ends up in
switch_fpu_return()
__fpregs_load_activate()
if (!fpregs_state_valid()) {
load_XSTATE_from_task();
}
But if nothing touched the FPU between T1 scheduling out and back in,
then the fpregs_state is still valid which means switch_fpu_return()
does nothing and just clears TIF_NEED_FPU_LOAD. Back to user space with
DEFAULT_PKRU loaded. -> FAIL #2!
The fix is simple: if get_xsave_addr() returns NULL then set the
PKRU value to 0 instead of the restrictive default PKRU value in
init_pkru_value.
[ bp: Massage in minor nitpicks from folks. ]
Fixes: 0cecca9d03c9 ("x86/fpu: Eager switch PKRU state")
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxxx>
Tested-by: Babu Moger <babu.moger@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20210608144346.045616965@xxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/include/asm/fpu/internal.h | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -579,9 +579,16 @@ static inline void switch_fpu_finish(str
* return to userland e.g. for a copy_to_user() operation.
*/
if (!(current->flags & PF_KTHREAD)) {
+ /*
+ * If the PKRU bit in xsave.header.xfeatures is not set,
+ * then the PKRU component was in init state, which means
+ * XRSTOR will set PKRU to 0. If the bit is not set then
+ * get_xsave_addr() will return NULL because the PKRU value
+ * in memory is not valid. This means pkru_val has to be
+ * set to 0 and not to init_pkru_value.
+ */
pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
- if (pk)
- pkru_val = pk->pkru;
+ pkru_val = pk ? pk->pkru : 0;
}
__write_pkru(pkru_val);
}
Patches currently in stable-queue which might be from tglx@xxxxxxxxxxxxx are
queue-5.12/x86-process-check-pf_kthread-and-not-current-mm-for-kernel-threads.patch
queue-5.12/x86-pkru-write-hardware-init-value-to-pkru-when-xstate-is-init.patch
queue-5.12/x86-fpu-prevent-state-corruption-in-__fpu__restore_sig.patch
queue-5.12/crash_core-vmcoreinfo-append-section_size_bits-to-vmcoreinfo.patch
queue-5.12/x86-fpu-invalidate-fpu-state-after-a-failed-xrstor-from-a-user-buffer.patch
queue-5.12/x86-fpu-reset-state-for-all-signal-restore-failures.patch
