set_fs() sets the addr_limit, which is used in access_ok() to
determine if an address is a user or kernel address.

Some code paths use set_fs() to temporarily elevate the addr_limit so
that kernel code can read/write kernel memory as if it were user
memory. That is fine as long as the code can't ever return to
userspace with the addr_limit still elevated.

If that did happen, then userspace can read/write kernel memory as if
it were user memory, eg. just with write(2). In case it's not clear,
that is very bad. It has also happened in the past due to bugs.

Commit 5ea0727b ("x86/syscalls: Check address limit on user-mode
return") added a mechanism to check the addr_limit value before
returning to userspace. Any call to set_fs() sets a thread flag,
TIF_FSCHECK, and if we see that on the return to userspace we go out
of line to check that the addr_limit value is not elevated.

For further info see the above commit, as well as:
  https://lwn.net/Articles/722267/
  https://bugs.chromium.org/p/project-zero/issues/detail?id=990

Verified to work on 64-bit Book3S using a POC that objdumps the system
call handler, and a modified lkdtm_CORRUPT_USER_DS() that doesn't kill
the caller.

Before:
  $ sudo ./test-tif-fscheck
  ...
  0000000000000000 <.data>:
         0:       e1 f7 8a 79     rldicl. r10,r12,30,63
         4:       80 03 82 40     bne     0x384
         8:       00 40 8a 71     andi.   r10,r12,16384
         c:       78 0b 2a 7c     mr      r10,r1
        10:       10 fd 21 38     addi    r1,r1,-752
        14:       08 00 c2 41     beq-    0x1c
        18:       58 09 2d e8     ld      r1,2392(r13)
        1c:       00 00 41 f9     std     r10,0(r1)
        20:       70 01 61 f9     std     r11,368(r1)
        24:       78 01 81 f9     std     r12,376(r1)
        28:       70 00 01 f8     std     r0,112(r1)
        2c:       78 00 41 f9     std     r10,120(r1)
        30:       20 00 82 41     beq     0x50
        34:       a6 42 4c 7d     mftb    r10

After:

  $ sudo ./test-tif-fscheck
  Killed

And in dmesg:
  Invalid address limit on user-mode return
  WARNING: CPU: 1 PID: 3689 at ../include/linux/syscalls.h:260 do_notify_resume+0x140/0x170
  ...
  NIP [c00000000001ee50] do_notify_resume+0x140/0x170
  LR [c00000000001ee4c] do_notify_resume+0x13c/0x170
  Call Trace:
    do_notify_resume+0x13c/0x170 (unreliable)
    ret_from_except_lite+0x70/0x74

Performance overhead is essentially zero in the usual case, because
the bit is checked as part of the existing _TIF_USER_WORK_MASK check.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>