Re: [PATCH] powerpc/64s: Don't use DSISR for SLB faults

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Michael Ellerman's message of February 22, 2022 9:34 pm:
> 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]
> 

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>

> 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>
> ---
>  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
> 
>