arm64: Add CONFIG_KUSER_HELPERS

Make it possible to disable the kuser helpers by adding a KUSER_HELPERS config option (enabled by default). When disabled, all kuser helpers-related code is removed from the kernel and no mapping is done at the fixed high address (0xffff0000); any attempt to use a kuser helper from a 32-bit process will result in a segfault.

Signed-off-by Mark Salyzyn <[email protected]>

v2:
- vectors_page and compat_vdso_spec as array of 2
- free sigpage if vectors allocation failed
- adjust makefile so one line for each of the assembler source modules
- split off assembler changes to a new previous patch in series to reduce churn
- modify slightly the feature documentation to reduce its reach
- modify slightly the feature documentation to rationalize the yes default.
- There are more ifdefs as a result of the rebase.

v3:
- rebase (minor conflicts)

Quoted 17 lines · Mark Salyzyn

Make it possible to disable the kuser helpers by adding a KUSER_HELPERS config option (enabled by default). When disabled, all kuser helpers-related code is removed from the kernel and no mapping is done at the fixed high address (0xffff0000); any attempt to use a kuser helper from a 32-bit process will result in a segfault.

Signed-off-by Mark Salyzyn <[email protected]>

v2:
- vectors_page and compat_vdso_spec as array of 2
- free sigpage if vectors allocation failed
- adjust makefile so one line for each of the assembler source modules
- split off assembler changes to a new previous patch in series to reduce churn
- modify slightly the feature documentation to reduce its reach
- modify slightly the feature documentation to rationalize the yes default.
- There are more ifdefs as a result of the rebase.

Quoted 2 lines

v3:
- rebase (minor conflicts)

This looks great to me. I'd love to see kuser helpers gone. :)

Catalin, Will, does this need anything else?

Quoted 20 lines · Kees Cook

Quoted 19 lines · Mark Salyzyn

Make it possible to disable the kuser helpers by adding a KUSER_HELPERS config option (enabled by default). When disabled, all kuser helpers-related code is removed from the kernel and no mapping is done at the fixed high address (0xffff0000); any attempt to use a kuser helper from a 32-bit process will result in a segfault.

Signed-off-by Mark Salyzyn <[email protected]>

v2:
- vectors_page and compat_vdso_spec as array of 2
- free sigpage if vectors allocation failed
- adjust makefile so one line for each of the assembler source modules
- split off assembler changes to a new previous patch in series to reduce churn
- modify slightly the feature documentation to reduce its reach
- modify slightly the feature documentation to rationalize the yes default.
- There are more ifdefs as a result of the rebase.

v3:
- rebase (minor conflicts)

Quoted 3 lines

This looks great to me. I'd love to see kuser helpers gone. :)

Catalin, Will, does this need anything else?

I haven't had chance to look at the patches yet (buried in other stuff), but I aim to get to it in the next couple of weeks.

Will