This is a note to let you know that I've just added the patch titled powerpc/64s: Don't use DSISR for SLB faults to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: powerpc-64s-don-t-use-dsisr-for-slb-faults.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit c570ad34483618428cb4592a6b0e8015c1177b56 Author: Michael Ellerman <mpe@xxxxxxxxxxxxxx> Date: Tue Feb 22 22:34:49 2022 +1100 powerpc/64s: Don't use DSISR for SLB faults [ 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@xxxxxxxxxxxxx> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> Reviewed-by: Nicholas Piggin <npiggin@xxxxxxxxx> Link: https://lore.kernel.org/r/20220222113449.319193-1-mpe@xxxxxxxxxxxxxx Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c index a8d0ce85d39a..4a15172dfef2 100644 --- a/arch/powerpc/mm/fault.c +++ b/arch/powerpc/mm/fault.c @@ -568,18 +568,24 @@ 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",