From: Thomas Gleixner tglx@linutronix.de
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@linutronix.de Signed-off-by: Borislav Petkov bp@suse.de Acked-by: Dave Hansen dave.hansen@linux.intel.com Acked-by: Rik van Riel riel@surriel.com Tested-by: Babu Moger babu.moger@amd.com Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20210608144346.045616965@linutronix.de Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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); }