From: Michael Ellerman mpe@ellerman.id.au
[ Upstream commit d4679ac8ea2e5078704aa1c026db36580cc1bf9a ]
Since commit 46ddcb3950a2 ("powerpc/mm: Show if a bad page fault on data is read or write.") we use page_fault_is_write(regs->dsisr) in __bad_page_fault() to determine if the fault is for a read or write, and change the message printed accordingly.
But SLB faults, aka Data Segment Interrupts, don't set DSISR (Data Storage Interrupt Status Register) to a useful value. All ISA versions from v2.03 through v3.1 specify that the Data Segment Interrupt sets DSISR "to an undefined value". As far as I can see there's no mention of SLB faults setting DSISR in any BookIV content either.
This manifests as accesses that should be a read being incorrectly reported as writes, for example, using the xmon "dump" command:
0:mon> d 0x5deadbeef0000000 5deadbeef0000000 [359526.415354][ C6] BUG: Unable to handle kernel data access on write at 0x5deadbeef0000000 [359526.415611][ C6] Faulting instruction address: 0xc00000000010a300 cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf400] pc: c00000000010a300: mread+0x90/0x190
If we disassemble the PC, we see a load instruction:
0:mon> di c00000000010a300 c00000000010a300 89490000 lbz r10,0(r9)
We can also see in exceptions-64s.S that the data_access_slb block doesn't set IDSISR=1, which means it doesn't load DSISR into pt_regs. So the value we're using to determine if the fault is a read/write is some stale value in pt_regs from a previous page fault.
Rework the printing logic to separate the SLB fault case out, and only print read/write in the cases where we can determine it.
The result looks like eg:
0:mon> d 0x5deadbeef0000000 5deadbeef0000000 [ 721.779525][ C6] BUG: Unable to handle kernel data access at 0x5deadbeef0000000 [ 721.779697][ C6] Faulting instruction address: 0xc00000000014cbe0 cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf390]
0:mon> d 0 0000000000000000 [ 742.793242][ C6] BUG: Kernel NULL pointer dereference at 0x00000000 [ 742.793316][ C6] Faulting instruction address: 0xc00000000014cbe0 cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf390]
Fixes: 46ddcb3950a2 ("powerpc/mm: Show if a bad page fault on data is read or write.") Reported-by: Nageswara R Sastry rnsastry@linux.ibm.com Signed-off-by: Michael Ellerman mpe@ellerman.id.au Reviewed-by: Nicholas Piggin npiggin@gmail.com Link: https://lore.kernel.org/r/20220222113449.319193-1-mpe@ellerman.id.au Signed-off-by: Sasha Levin sashal@kernel.org --- arch/powerpc/mm/fault.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-)
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c index eb8ecd7343a9..7ba6d3eff636 100644 --- a/arch/powerpc/mm/fault.c +++ b/arch/powerpc/mm/fault.c @@ -567,18 +567,24 @@ NOKPROBE_SYMBOL(hash__do_page_fault); static void __bad_page_fault(struct pt_regs *regs, int sig) { int is_write = page_fault_is_write(regs->dsisr); + const char *msg;
/* kernel has accessed a bad area */
+ if (regs->dar < PAGE_SIZE) + msg = "Kernel NULL pointer dereference"; + else + msg = "Unable to handle kernel data access"; + switch (TRAP(regs)) { case INTERRUPT_DATA_STORAGE: - case INTERRUPT_DATA_SEGMENT: case INTERRUPT_H_DATA_STORAGE: - pr_alert("BUG: %s on %s at 0x%08lx\n", - regs->dar < PAGE_SIZE ? "Kernel NULL pointer dereference" : - "Unable to handle kernel data access", + pr_alert("BUG: %s on %s at 0x%08lx\n", msg, is_write ? "write" : "read", regs->dar); break; + case INTERRUPT_DATA_SEGMENT: + pr_alert("BUG: %s at 0x%08lx\n", msg, regs->dar); + break; case INTERRUPT_INST_STORAGE: case INTERRUPT_INST_SEGMENT: pr_alert("BUG: Unable to handle kernel instruction fetch%s",