On Thu, 26 Jul 2018 22:42:44 +1000 Michael Ellerman <m...@ellerman.id.au> wrote:
> Because rfi_flush_fallback runs immediately before the return to > userspace it currently runs with the user r1 (stack pointer). This > means if we oops in there we will report a bad kernel stack pointer in > the exception entry path, eg: > > Bad kernel stack pointer 7ffff7150e40 at c0000000000023b4 > Oops: Bad kernel stack pointer, sig: 6 [#1] > LE SMP NR_CPUS=32 NUMA PowerNV > Modules linked in: > CPU: 0 PID: 1246 Comm: klogd Not tainted > 4.18.0-rc2-gcc-7.3.1-00175-g0443f8a69ba3 #7 > NIP: c0000000000023b4 LR: 0000000010053e00 CTR: 0000000000000040 > REGS: c0000000fffe7d40 TRAP: 4100 Not tainted > (4.18.0-rc2-gcc-7.3.1-00175-g0443f8a69ba3) > MSR: 9000000002803031 <SF,HV,VEC,VSX,FP,ME,IR,DR,LE> CR: 44000442 XER: > 20000000 > CFAR: c00000000000bac8 IRQMASK: c0000000f1e66a80 > GPR00: 0000000002000000 00007ffff7150e40 00007fff93a99900 0000000000000020 > ... > NIP [c0000000000023b4] rfi_flush_fallback+0x34/0x80 > LR [0000000010053e00] 0x10053e00 > > Although the NIP tells us where we were, and the TRAP number tells us > what happened, it would still be nicer if we could report the actual > exception rather than barfing about the stack pointer. > > We an do that fairly simply by loading the kernel stack pointer on > entry and restoring the user value before returning. That way we see a > regular oops such as: > > Unrecoverable exception 4100 at c00000000000239c > Oops: Unrecoverable exception, sig: 6 [#1] > LE SMP NR_CPUS=32 NUMA PowerNV > Modules linked in: > CPU: 0 PID: 1251 Comm: klogd Not tainted > 4.18.0-rc3-gcc-7.3.1-00097-g4ebfcac65acd-dirty #40 > NIP: c00000000000239c LR: 0000000010053e00 CTR: 0000000000000040 > REGS: c0000000f1e17bb0 TRAP: 4100 Not tainted > (4.18.0-rc3-gcc-7.3.1-00097-g4ebfcac65acd-dirty) > MSR: 9000000002803031 <SF,HV,VEC,VSX,FP,ME,IR,DR,LE> CR: 44000442 XER: > 20000000 > CFAR: c00000000000bac8 IRQMASK: 0 > ... > NIP [c00000000000239c] rfi_flush_fallback+0x3c/0x80 > LR [0000000010053e00] 0x10053e00 > Call Trace: > [c0000000f1e17e30] [c00000000000b9e4] system_call+0x5c/0x70 (unreliable) > > Note this shouldn't make the kernel stack pointer vulnerable to a > meltdown attack, because it should be flushed from the cache before we > return to userspace. The user r1 value will be in the cache, because > we load it in the return path, but that is harmless. > > Signed-off-by: Michael Ellerman <m...@ellerman.id.au> Yeah that's a lot nicer, thanks. Reviewed-by: Nicholas Piggin <npig...@gmail.com> > --- > arch/powerpc/kernel/exceptions-64s.S | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/arch/powerpc/kernel/exceptions-64s.S > b/arch/powerpc/kernel/exceptions-64s.S > index f7cc12aa3dc6..a6fa85916273 100644 > --- a/arch/powerpc/kernel/exceptions-64s.S > +++ b/arch/powerpc/kernel/exceptions-64s.S > @@ -1518,6 +1518,8 @@ TRAMP_REAL_BEGIN(stf_barrier_fallback) > TRAMP_REAL_BEGIN(rfi_flush_fallback) > SET_SCRATCH0(r13); > GET_PACA(r13); > + std r1,PACA_EXRFI+EX_R12(r13) > + ld r1,PACAKSAVE(r13) > std r9,PACA_EXRFI+EX_R9(r13) > std r10,PACA_EXRFI+EX_R10(r13) > std r11,PACA_EXRFI+EX_R11(r13) > @@ -1552,12 +1554,15 @@ TRAMP_REAL_BEGIN(rfi_flush_fallback) > ld r9,PACA_EXRFI+EX_R9(r13) > ld r10,PACA_EXRFI+EX_R10(r13) > ld r11,PACA_EXRFI+EX_R11(r13) > + ld r1,PACA_EXRFI+EX_R12(r13) > GET_SCRATCH0(r13); > rfid > > TRAMP_REAL_BEGIN(hrfi_flush_fallback) > SET_SCRATCH0(r13); > GET_PACA(r13); > + std r1,PACA_EXRFI+EX_R12(r13) > + ld r1,PACAKSAVE(r13) > std r9,PACA_EXRFI+EX_R9(r13) > std r10,PACA_EXRFI+EX_R10(r13) > std r11,PACA_EXRFI+EX_R11(r13) > @@ -1592,6 +1597,7 @@ TRAMP_REAL_BEGIN(hrfi_flush_fallback) > ld r9,PACA_EXRFI+EX_R9(r13) > ld r10,PACA_EXRFI+EX_R10(r13) > ld r11,PACA_EXRFI+EX_R11(r13) > + ld r1,PACA_EXRFI+EX_R12(r13) > GET_SCRATCH0(r13); > hrfid >
