4.14-stable review patch. If anyone has any objections, please let me know.
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From: Timothy Pearson tpearson@raptorengineering.com
commit 5e1d824f9a283cbf90f25241b66d1f69adb3835b upstream.
During floating point and vector save to thread data f0/vs0 are clobbered by the FPSCR/VSCR store routine. This has been obvserved to lead to userspace register corruption and application data corruption with io-uring.
Fix it by restoring f0/vs0 after FPSCR/VSCR store has completed for all the FP, altivec, VMX register save paths.
Tested under QEMU in kvm mode, running on a Talos II workstation with dual POWER9 DD2.2 CPUs.
Additional detail (mpe):
Typically save_fpu() is called from __giveup_fpu() which saves the FP regs and also *turns off FP* in the tasks MSR, meaning the kernel will reload the FP regs from the thread struct before letting the task use FP again. So in that case save_fpu() is free to clobber f0 because the FP regs no longer hold live values for the task.
There is another case though, which is the path via: sys_clone() ... copy_process() dup_task_struct() arch_dup_task_struct() flush_all_to_thread() save_all()
That path saves the FP regs but leaves them live. That's meant as an optimisation for a process that's using FP/VSX and then calls fork(), leaving the regs live means the parent process doesn't have to take a fault after the fork to get its FP regs back. The optimisation was added in commit 8792468da5e1 ("powerpc: Add the ability to save FPU without giving it up").
That path does clobber f0, but f0 is volatile across function calls, and typically programs reach copy_process() from userspace via a syscall wrapper function. So in normal usage f0 being clobbered across a syscall doesn't cause visible data corruption.
But there is now a new path, because io-uring can call copy_process() via create_io_thread() from the signal handling path. That's OK if the signal is handled as part of syscall return, but it's not OK if the signal is handled due to some other interrupt.
That path is:
interrupt_return_srr_user() interrupt_exit_user_prepare() interrupt_exit_user_prepare_main() do_notify_resume() get_signal() task_work_run() create_worker_cb() create_io_worker() copy_process() dup_task_struct() arch_dup_task_struct() flush_all_to_thread() save_all() if (tsk->thread.regs->msr & MSR_FP) save_fpu() # f0 is clobbered and potentially live in userspace
Note the above discussion applies equally to save_altivec().
Fixes: 8792468da5e1 ("powerpc: Add the ability to save FPU without giving it up") Cc: stable@vger.kernel.org # v4.6+ Closes: https://lore.kernel.org/all/480932026.45576726.1699374859845.JavaMail.zimbra... Closes: https://lore.kernel.org/linuxppc-dev/480221078.47953493.1700206777956.JavaMa... Tested-by: Timothy Pearson tpearson@raptorengineering.com Tested-by: Jens Axboe axboe@kernel.dk Signed-off-by: Timothy Pearson tpearson@raptorengineering.com [mpe: Reword change log to describe exact path of corruption & other minor tweaks] Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://msgid.link/1921539696.48534988.1700407082933.JavaMail.zimbra@raptore... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/powerpc/kernel/fpu.S | 13 +++++++++++++ arch/powerpc/kernel/vector.S | 2 ++ 2 files changed, 15 insertions(+)
--- a/arch/powerpc/kernel/fpu.S +++ b/arch/powerpc/kernel/fpu.S @@ -27,6 +27,15 @@ #include <asm/export.h>
#ifdef CONFIG_VSX +#define __REST_1FPVSR(n,c,base) \ +BEGIN_FTR_SECTION \ + b 2f; \ +END_FTR_SECTION_IFSET(CPU_FTR_VSX); \ + REST_FPR(n,base); \ + b 3f; \ +2: REST_VSR(n,c,base); \ +3: + #define __REST_32FPVSRS(n,c,base) \ BEGIN_FTR_SECTION \ b 2f; \ @@ -45,9 +54,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_VSX); 2: SAVE_32VSRS(n,c,base); \ 3: #else +#define __REST_1FPVSR(n,b,base) REST_FPR(n, base) #define __REST_32FPVSRS(n,b,base) REST_32FPRS(n, base) #define __SAVE_32FPVSRS(n,b,base) SAVE_32FPRS(n, base) #endif +#define REST_1FPVSR(n,c,base) __REST_1FPVSR(n,__REG_##c,__REG_##base) #define REST_32FPVSRS(n,c,base) __REST_32FPVSRS(n,__REG_##c,__REG_##base) #define SAVE_32FPVSRS(n,c,base) __SAVE_32FPVSRS(n,__REG_##c,__REG_##base)
@@ -70,6 +81,7 @@ _GLOBAL(store_fp_state) SAVE_32FPVSRS(0, R4, R3) mffs fr0 stfd fr0,FPSTATE_FPSCR(r3) + REST_1FPVSR(0, R4, R3) blr EXPORT_SYMBOL(store_fp_state)
@@ -134,6 +146,7 @@ _GLOBAL(save_fpu) 2: SAVE_32FPVSRS(0, R4, R6) mffs fr0 stfd fr0,FPSTATE_FPSCR(r6) + REST_1FPVSR(0, R4, R6) blr
/* --- a/arch/powerpc/kernel/vector.S +++ b/arch/powerpc/kernel/vector.S @@ -30,6 +30,7 @@ _GLOBAL(store_vr_state) mfvscr v0 li r4, VRSTATE_VSCR stvx v0, r4, r3 + lvx v0, 0, r3 blr EXPORT_SYMBOL(store_vr_state)
@@ -100,6 +101,7 @@ _GLOBAL(save_altivec) mfvscr v0 li r4,VRSTATE_VSCR stvx v0,r4,r7 + lvx v0,0,r7 blr
#ifdef CONFIG_VSX