• James Morse's avatar
    arm64: kernel: Don't toggle PAN on systems with UAO · 70544196
    James Morse authored
    If a CPU supports both Privileged Access Never (PAN) and User Access
    Override (UAO), we don't need to disable/re-enable PAN round all
    copy_to_user() like calls.
    
    UAO alternatives cause these calls to use the 'unprivileged' load/store
    instructions, which are overridden to be the privileged kind when
    fs==KERNEL_DS.
    
    This patch changes the copy_to_user() calls to have their PAN toggling
    depend on a new composite 'feature' ARM64_ALT_PAN_NOT_UAO.
    
    If both features are detected, PAN will be enabled, but the copy_to_user()
    alternatives will not be applied. This means PAN will be enabled all the
    time for these functions. If only PAN is detected, the toggling will be
    enabled as normal.
    
    This will save the time taken to disable/re-enable PAN, and allow us to
    catch copy_to_user() accesses that occur with fs==KERNEL_DS.
    
    Futex and swp-emulation code continue to hang their PAN toggling code on
    ARM64_HAS_PAN.
    Signed-off-by: default avatarJames Morse <james.morse@arm.com>
    Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
    70544196
cpufeature.c 32.1 KB