From: Gustavo Romero gromero@linux.ibm.com
commit a8318c13e79badb92bc6640704a64cc022a6eb97 upstream.
When in userspace and MSR FP=0 the hardware FP state is unrelated to the current process. This is extended for transactions where if tbegin is run with FP=0, the hardware checkpoint FP state will also be unrelated to the current process. Due to this, we need to ensure this hardware checkpoint is updated with the correct state before we enable FP for this process.
Unfortunately we get this wrong when returning to a process from a hardware interrupt. A process that starts a transaction with FP=0 can take an interrupt. When the kernel returns back to that process, we change to FP=1 but with hardware checkpoint FP state not updated. If this transaction is then rolled back, the FP registers now contain the wrong state.
The process looks like this: Userspace: Kernel
Start userspace with MSR FP=0 TM=1 < ----- ... tbegin bne Hardware interrupt ---- > <do_IRQ...> .... ret_from_except restore_math() /* sees FP=0 */ restore_fp() tm_active_with_fp() /* sees FP=1 (Incorrect) */ load_fp_state() FP = 0 -> 1 < ----- Return to userspace with MSR TM=1 FP=1 with junk in the FP TM checkpoint TM rollback reads FP junk
When returning from the hardware exception, tm_active_with_fp() is incorrectly making restore_fp() call load_fp_state() which is setting FP=1.
The fix is to remove tm_active_with_fp().
tm_active_with_fp() is attempting to handle the case where FP state has been changed inside a transaction. In this case the checkpointed and transactional FP state is different and hence we must restore the FP state (ie. we can't do lazy FP restore inside a transaction that's used FP). It's safe to remove tm_active_with_fp() as this case is handled by restore_tm_state(). restore_tm_state() detects if FP has been using inside a transaction and will set load_fp and call restore_math() to ensure the FP state (checkpoint and transaction) is restored.
This is a data integrity problem for the current process as the FP registers are corrupted. It's also a security problem as the FP registers from one process may be leaked to another.
Similarly for VMX.
A simple testcase to replicate this will be posted to tools/testing/selftests/powerpc/tm/tm-poison.c
This fixes CVE-2019-15031.
Fixes: a7771176b439 ("powerpc: Don't enable FP/Altivec if not checkpointed") Cc: stable@vger.kernel.org # 4.15+ Signed-off-by: Gustavo Romero gromero@linux.ibm.com Signed-off-by: Michael Neuling mikey@neuling.org Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20190904045529.23002-2-gromero@linux.vnet.ibm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/powerpc/kernel/process.c | 18 ++---------------- 1 file changed, 2 insertions(+), 16 deletions(-)
--- a/arch/powerpc/kernel/process.c +++ b/arch/powerpc/kernel/process.c @@ -101,21 +101,8 @@ static void check_if_tm_restore_required } }
-static bool tm_active_with_fp(struct task_struct *tsk) -{ - return MSR_TM_ACTIVE(tsk->thread.regs->msr) && - (tsk->thread.ckpt_regs.msr & MSR_FP); -} - -static bool tm_active_with_altivec(struct task_struct *tsk) -{ - return MSR_TM_ACTIVE(tsk->thread.regs->msr) && - (tsk->thread.ckpt_regs.msr & MSR_VEC); -} #else static inline void check_if_tm_restore_required(struct task_struct *tsk) { } -static inline bool tm_active_with_fp(struct task_struct *tsk) { return false; } -static inline bool tm_active_with_altivec(struct task_struct *tsk) { return false; } #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
bool strict_msr_control; @@ -252,7 +239,7 @@ EXPORT_SYMBOL(enable_kernel_fp);
static int restore_fp(struct task_struct *tsk) { - if (tsk->thread.load_fp || tm_active_with_fp(tsk)) { + if (tsk->thread.load_fp) { load_fp_state(¤t->thread.fp_state); current->thread.load_fp++; return 1; @@ -334,8 +321,7 @@ EXPORT_SYMBOL_GPL(flush_altivec_to_threa
static int restore_altivec(struct task_struct *tsk) { - if (cpu_has_feature(CPU_FTR_ALTIVEC) && - (tsk->thread.load_vec || tm_active_with_altivec(tsk))) { + if (cpu_has_feature(CPU_FTR_ALTIVEC) && (tsk->thread.load_vec)) { load_vr_state(&tsk->thread.vr_state); tsk->thread.used_vr = 1; tsk->thread.load_vec++;