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    SubjectRe: [PATCH RESEND 1/4] uaccess: Add user_read_access_begin/end and user_write_access_begin/end
    On Thu, Apr 02, 2020 at 07:03:28PM +0200, Christophe Leroy wrote:

    > user_access_begin() grants both read and write.
    >
    > This patch adds user_read_access_begin() and user_write_access_begin() but
    > it doesn't remove user_access_begin()

    Ouch... So the most generic name is for the rarest case?

    > > What should we do about that? Do we prohibit such blocks outside
    > > of arch?
    > >
    > > What should we do about arm and s390? There we want a cookie passed
    > > from beginning of block to its end; should that be a return value?
    >
    > That was the way I implemented it in January, see
    > https://patchwork.ozlabs.org/patch/1227926/
    >
    > There was some discussion around that and most noticeable was:
    >
    > H. Peter (hpa) said about it: "I have *deep* concern with carrying state in
    > a "key" variable: it's a direct attack vector for a crowbar attack,
    > especially since it is by definition live inside a user access region."

    > This patch minimises the change by just adding user_read_access_begin() and
    > user_write_access_begin() keeping the same parameters as the existing
    > user_access_begin().

    Umm... What about the arm situation? The same concerns would apply there,
    wouldn't they? Currently we have
    static __always_inline unsigned int uaccess_save_and_enable(void)
    {
    #ifdef CONFIG_CPU_SW_DOMAIN_PAN
    unsigned int old_domain = get_domain();

    /* Set the current domain access to permit user accesses */
    set_domain((old_domain & ~domain_mask(DOMAIN_USER)) |
    domain_val(DOMAIN_USER, DOMAIN_CLIENT));

    return old_domain;
    #else
    return 0;
    #endif
    }
    and
    static __always_inline void uaccess_restore(unsigned int flags)
    {
    #ifdef CONFIG_CPU_SW_DOMAIN_PAN
    /* Restore the user access mask */
    set_domain(flags);
    #endif
    }

    How much do we need nesting on those, anyway? rmk?

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