From: Michael Ellerman mpe@ellerman.id.au
[ Upstream commit 7ffa8b7dc11752827329e4e84a574ea6aaf24716 ]
Aneesh increased the size of struct pt_regs by 16 bytes and started seeing this WARN_ON:
smp: Bringing up secondary CPUs ... ------------[ cut here ]------------ WARNING: CPU: 0 PID: 0 at arch/powerpc/kernel/process.c:455 giveup_all+0xb4/0x110 Modules linked in: CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.7.0-rc2-gcc-8.2.0-1.g8f6a41f-default+ #318 NIP: c00000000001a2b4 LR: c00000000001a29c CTR: c0000000031d0000 REGS: c0000000026d3980 TRAP: 0700 Not tainted (5.7.0-rc2-gcc-8.2.0-1.g8f6a41f-default+) MSR: 800000000282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE> CR: 48048224 XER: 00000000 CFAR: c000000000019cc8 IRQMASK: 1 GPR00: c00000000001a264 c0000000026d3c20 c0000000026d7200 800000000280b033 GPR04: 0000000000000001 0000000000000000 0000000000000077 30206d7372203164 GPR08: 0000000000002000 0000000002002000 800000000280b033 3230303030303030 GPR12: 0000000000008800 c0000000031d0000 0000000000800050 0000000002000066 GPR16: 000000000309a1a0 000000000309a4b0 000000000309a2d8 000000000309a890 GPR20: 00000000030d0098 c00000000264da40 00000000fd620000 c0000000ff798080 GPR24: c00000000264edf0 c0000001007469f0 00000000fd620000 c0000000020e5e90 GPR28: c00000000264edf0 c00000000264d200 000000001db60000 c00000000264d200 NIP [c00000000001a2b4] giveup_all+0xb4/0x110 LR [c00000000001a29c] giveup_all+0x9c/0x110 Call Trace: [c0000000026d3c20] [c00000000001a264] giveup_all+0x64/0x110 (unreliable) [c0000000026d3c90] [c00000000001ae34] __switch_to+0x104/0x480 [c0000000026d3cf0] [c000000000e0b8a0] __schedule+0x320/0x970 [c0000000026d3dd0] [c000000000e0c518] schedule_idle+0x38/0x70 [c0000000026d3df0] [c00000000019c7c8] do_idle+0x248/0x3f0 [c0000000026d3e70] [c00000000019cbb8] cpu_startup_entry+0x38/0x40 [c0000000026d3ea0] [c000000000011bb0] rest_init+0xe0/0xf8 [c0000000026d3ed0] [c000000002004820] start_kernel+0x990/0x9e0 [c0000000026d3f90] [c00000000000c49c] start_here_common+0x1c/0x400
Which was unexpected. The warning is checking the thread.regs->msr value of the task we are switching from:
usermsr = tsk->thread.regs->msr; ... WARN_ON((usermsr & MSR_VSX) && !((usermsr & MSR_FP) && (usermsr & MSR_VEC)));
ie. if MSR_VSX is set then both of MSR_FP and MSR_VEC are also set.
Dumping tsk->thread.regs->msr we see that it's: 0x1db60000
Which is not a normal looking MSR, in fact the only valid bit is MSR_VSX, all the other bits are reserved in the current definition of the MSR.
We can see from the oops that it was swapper/0 that we were switching from when we hit the warning, ie. init_task. So its thread.regs points to the base (high addresses) in init_stack.
Dumping the content of init_task->thread.regs, with the members of pt_regs annotated (the 16 bytes larger version), we see:
0000000000000000 c000000002780080 gpr[0] gpr[1] 0000000000000000 c000000002666008 gpr[2] gpr[3] c0000000026d3ed0 0000000000000078 gpr[4] gpr[5] c000000000011b68 c000000002780080 gpr[6] gpr[7] 0000000000000000 0000000000000000 gpr[8] gpr[9] c0000000026d3f90 0000800000002200 gpr[10] gpr[11] c000000002004820 c0000000026d7200 gpr[12] gpr[13] 000000001db60000 c0000000010aabe8 gpr[14] gpr[15] c0000000010aabe8 c0000000010aabe8 gpr[16] gpr[17] c00000000294d598 0000000000000000 gpr[18] gpr[19] 0000000000000000 0000000000001ff8 gpr[20] gpr[21] 0000000000000000 c00000000206d608 gpr[22] gpr[23] c00000000278e0cc 0000000000000000 gpr[24] gpr[25] 000000002fff0000 c000000000000000 gpr[26] gpr[27] 0000000002000000 0000000000000028 gpr[28] gpr[29] 000000001db60000 0000000004750000 gpr[30] gpr[31] 0000000002000000 000000001db60000 nip msr 0000000000000000 0000000000000000 orig_r3 ctr c00000000000c49c 0000000000000000 link xer 0000000000000000 0000000000000000 ccr softe 0000000000000000 0000000000000000 trap dar 0000000000000000 0000000000000000 dsisr result 0000000000000000 0000000000000000 ppr kuap 0000000000000000 0000000000000000 pad[2] pad[3]
This looks suspiciously like stack frames, not a pt_regs. If we look closely we can see return addresses from the stack trace above, c000000002004820 (start_kernel) and c00000000000c49c (start_here_common).
init_task->thread.regs is setup at build time in processor.h:
#define INIT_THREAD { \ .ksp = INIT_SP, \ .regs = (struct pt_regs *)INIT_SP - 1, /* XXX bogus, I think */ \
The early boot code where we setup the initial stack is:
LOAD_REG_ADDR(r3,init_thread_union)
/* set up a stack pointer */ LOAD_REG_IMMEDIATE(r1,THREAD_SIZE) add r1,r3,r1 li r0,0 stdu r0,-STACK_FRAME_OVERHEAD(r1)
Which creates a stack frame of size 112 bytes (STACK_FRAME_OVERHEAD). Which is far too small to contain a pt_regs.
So the result is init_task->thread.regs is pointing at some stack frames on the init stack, not at a pt_regs.
We have gotten away with this for so long because with pt_regs at its current size the MSR happens to point into the first frame, at a location that is not written to by the early asm. With the 16 byte expansion the MSR falls into the second frame, which is used by the compiler, and collides with a saved register that tends to be non-zero.
As far as I can see this has been wrong since the original merge of 64-bit ppc support, back in 2002.
Conceptually swapper should have no regs, it never entered from userspace, and in fact that's what we do on 32-bit. It's also presumably what the "bogus" comment is referring to.
So I think the right fix is to just not-initialise regs at all. I'm slightly worried this will break some code that isn't prepared for a NULL regs, but we'll have to see.
Remove the comment in head_64.S which refers to us setting up the regs (even though we never did), and is otherwise not really accurate any more.
Reported-by: Aneesh Kumar K.V aneesh.kumar@linux.ibm.com Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20200428123130.73078-1-mpe@ellerman.id.au Signed-off-by: Sasha Levin sashal@kernel.org --- arch/powerpc/include/asm/processor.h | 1 - arch/powerpc/kernel/head_64.S | 9 +-------- 2 files changed, 1 insertion(+), 9 deletions(-)
diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h index eedcbfb9a6ff3..c220cb9eccad9 100644 --- a/arch/powerpc/include/asm/processor.h +++ b/arch/powerpc/include/asm/processor.h @@ -301,7 +301,6 @@ struct thread_struct { #else #define INIT_THREAD { \ .ksp = INIT_SP, \ - .regs = (struct pt_regs *)INIT_SP - 1, /* XXX bogus, I think */ \ .addr_limit = KERNEL_DS, \ .fpexc_mode = 0, \ .fscr = FSCR_TAR | FSCR_EBB \ diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S index ddfbd02140d9b..0e05a9a47a4b3 100644 --- a/arch/powerpc/kernel/head_64.S +++ b/arch/powerpc/kernel/head_64.S @@ -947,15 +947,8 @@ start_here_multiplatform: std r0,0(r4) #endif
- /* The following gets the stack set up with the regs */ - /* pointing to the real addr of the kernel stack. This is */ - /* all done to support the C function call below which sets */ - /* up the htab. This is done because we have relocated the */ - /* kernel but are still running in real mode. */ - - LOAD_REG_ADDR(r3,init_thread_union) - /* set up a stack pointer */ + LOAD_REG_ADDR(r3,init_thread_union) LOAD_REG_IMMEDIATE(r1,THREAD_SIZE) add r1,r3,r1 li r0,0