x86/syscalls: Check address limit on user-mode return

Ensure the address limit is a user-mode segment before returning to
user-mode. Otherwise a process can corrupt kernel-mode memory and elevate
privileges [1].

The set_fs function sets the TIF_SETFS flag to force a slow path on
return. In the slow path, the address limit is checked to be USER_DS if
needed.

The addr_limit_user_check function is added as a cross-architecture
function to check the address limit.

[1] https://bugs.chromium.org/p/project-zero/issues/detail?id=990

Signed-off-by: Thomas Garnier <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: [email protected]
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: David Howells <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Miroslav Benes <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Pratyush Anand <[email protected]>
Cc: Russell King <[email protected]>
Cc: Petr Mladek <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: [email protected]
Cc: Will Drewry <[email protected]>
Cc: [email protected]
Cc: Oleg Nesterov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]

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