• Christophe Leroy's avatar
    powerpc/32s: Fix DSI and ISI exceptions for CONFIG_VMAP_STACK · 232ca1ee
    Christophe Leroy authored
    hash_page() needs to read page tables from kernel memory. When entire
    kernel memory is mapped by BATs, which is normally the case when
    CONFIG_STRICT_KERNEL_RWX is not set, it works even if the page hosting
    the page table is not referenced in the MMU hash table.
    
    However, if the page where the page table resides is not covered by
    a BAT, a DSI fault can be encountered from hash_page(), and it loops
    forever. This can happen when CONFIG_STRICT_KERNEL_RWX is selected
    and the alignment of the different regions is too small to allow
    covering the entire memory with BATs. This also happens when
    CONFIG_DEBUG_PAGEALLOC is selected or when booting with 'nobats'
    flag.
    
    Also, if the page containing the kernel stack is not present in the
    MMU hash table, registers cannot be saved and a recursive DSI fault
    is encountered.
    
    To allow hash_page() to properly do its job at all time and load the
    MMU hash table whenever needed, it must run with data MMU disabled.
    This means it must be called before re-enabling data MMU. To allow
    this, registers clobbered by hash_page() and create_hpte() have to
    be saved in the thread struct together with SRR0, SSR1, DAR and DSISR.
    It is also necessary to ensure that DSI prolog doesn't overwrite
    regs saved by prolog of the current running exception. That means:
    - DSI can only use SPRN_SPRG_SCRATCH0
    - Exceptions must free SPRN_SPRG_SCRATCH0 before writing to the stack.
    
    This also fixes the Oops reported by Erhard when create_hpte() is
    called by add_hash_page().
    
    Due to prolog size increase, a few more exceptions had to get split
    in two parts.
    
    Fixes: cd08f109 ("powerpc/32s: Enable CONFIG_VMAP_STACK")
    Reported-by: default avatarErhard F. <erhard_f@mailbox.org>
    Signed-off-by: default avatarChristophe Leroy <christophe.leroy@c-s.fr>
    Tested-by: default avatarErhard F. <erhard_f@mailbox.org>
    Tested-by: default avatarLarry Finger <Larry.Finger@lwfinger.net>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Link: https://bugzilla.kernel.org/show_bug.cgi?id=206501
    Link: https://lore.kernel.org/r/64a4aa44686e9fd4b01333401367029771d9b231.1581761633.git.christophe.leroy@c-s.fr
    232ca1ee
head_32.h 8.05 KB