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    SubjectRe: [PATCH v1 1/5] powerpc/mm: sanity_check_fault() should work for all, not only BOOK3S
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    Le 08/09/2020 à 10:43, Nicholas Piggin a écrit :
    > Excerpts from Christophe Leroy's message of August 7, 2020 3:15 am:
    >> The verification and message introduced by commit 374f3f5979f9
    >> ("powerpc/mm/hash: Handle user access of kernel address gracefully")
    >> applies to all platforms, it should not be limited to BOOK3S.
    >>
    >> Make the BOOK3S version of sanity_check_fault() the one for all,
    >> and bail out earlier if not BOOK3S.
    >>
    >> Fixes: 374f3f5979f9 ("powerpc/mm/hash: Handle user access of kernel address gracefully")
    >> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
    >> ---
    >> arch/powerpc/mm/fault.c | 8 +++-----
    >> 1 file changed, 3 insertions(+), 5 deletions(-)
    >>
    >> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
    >> index 925a7231abb3..2efa34d7e644 100644
    >> --- a/arch/powerpc/mm/fault.c
    >> +++ b/arch/powerpc/mm/fault.c
    >> @@ -303,7 +303,6 @@ static inline void cmo_account_page_fault(void)
    >> static inline void cmo_account_page_fault(void) { }
    >> #endif /* CONFIG_PPC_SMLPAR */
    >>
    >> -#ifdef CONFIG_PPC_BOOK3S
    >> static void sanity_check_fault(bool is_write, bool is_user,
    >> unsigned long error_code, unsigned long address)
    >> {
    >> @@ -320,6 +319,9 @@ static void sanity_check_fault(bool is_write, bool is_user,
    >> return;
    >> }
    >>
    >> + if (!IS_ENABLED(CONFIG_PPC_BOOK3S))
    >> + return;
    >
    > Seems okay. Why is address == -1 special though? I guess it's because
    > it may not be an exploit kernel reference but a buggy pointer underflow?
    > In that case -1 doesn't seem like it would catch very much. Would it be
    > better to test for high bit set for example ((long)address < 0) ?

    See
    https://github.com/linuxppc/linux/commit/0f9aee0cb9da7db7d96f63cfa2dc5e4f1bffeb87#diff-f9658f412252f3bb3093e0a95b37f3ac

    -1 is what mmap() returns on error, if the app uses that as a pointer
    that's a programming error not an exploit.

    Euh .. If you test (long)address < 0, then the entire kernel falls into
    that range as usually it goes from 0xc0000000 to 0xffffffff

    But we could skip the top page entirely, anyway it is never mapped.

    >
    > Anyway for your patch
    >
    > Reviewed-by: Nicholas Piggin <npiggin@gmail.com>

    Thanks
    Christophe

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