Subject: Re: [PATCH] arm64: mm: avoid virt_to_phys(init_mm.pgd)

From: Catalin Marinas <[email-masked]>
Date: Thu, 3 Oct 2019 16:40:02 +0100

On Thu, Oct 03, 2019 at 10:49:32AM +0100, Mark Rutland wrote:
> If we take an unhandled fault in the kernel, we call show_pte() to dump
> the {PGDP,PGD,PUD,PMD,PTE} values for the corresponding page table walk,
> where the PGDP value is virt_to_phys(mm->pgd).
> 
> The boot-time and runtime kernel page tables, init_pg_dir and
> swapper_pg_dir respectively, are kernel symbols. Thus, it is not valid
> to call virt_to_phys() on either of these, though we'll do so if we take
> a fault on a TTBR1 address.
> 
> When CONFIG_DEBUG_VIRTUAL is not selected, virt_to_phys() will silently
> fix this up. However, when CONFIG_DEBUG_VIRTUAL is selected, this
> results in splats as below. Depending on when these occur, they can
> happen to suppress information needed to debug the original unhandled
> fault, such as the backtrace:
> 
> | Unable to handle kernel paging request at virtual address ffff7fffec73cf0f
> | Mem abort info:
> |   ESR = 0x96000004
> |   EC = 0x25: DABT (current EL), IL = 32 bits
> |   SET = 0, FnV = 0
> |   EA = 0, S1PTW = 0
> | Data abort info:
> |   ISV = 0, ISS = 0x00000004
> |   CM = 0, WnR = 0
> | ------------[ cut here ]------------
> | virt_to_phys used for non-linear address: 00000000102c9dbe (swapper_pg_dir+0x0/0x1000)
> | WARNING: CPU: 1 PID: 7558 at arch/arm64/mm/physaddr.c:15 __virt_to_phys+0xe0/0x170 arch/arm64/mm/physaddr.c:12
> | Kernel panic - not syncing: panic_on_warn set ...
> | SMP: stopping secondary CPUs
> | Dumping ftrace buffer:
> |    (ftrace buffer empty)
> | Kernel Offset: disabled
> | CPU features: 0x0002,23000438
> | Memory Limit: none
> | Rebooting in 1 seconds..
> 
> We can avoid this by ensuring that we call __pa_symbol() for
> init_mm.pgd, as this will always be a kernel symbol. As the dumped
> {PGD,PUD,PMD,PTE} values are the raw values from the relevant entries we
> don't need to handle these specially.
> 
> Signed-off-by: Mark Rutland <[email-masked]>
> Cc: Catalin Marinas <[email-masked]>
> Cc: James Morse <[email-masked]>
> Cc: Will Deacon <[email-masked]>

Acked-by: Catalin Marinas <[email-masked]>

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