[PATCH 5.17 0641/1126] powerpc/64s: Dont use DSISR for SLB faults

Greg Kroah-Hartman
2022-04-05T10:04:19Z

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",
--
2.34.1