With CONFIG_CFI_CLANG, the compiler injects a type preamble immediately
before each function and a check to validate the target function type
before indirect calls:

  ; type preamble
    .word <id>
  function:
    ...
  ; indirect call check
    lw      t1, -4(a0)
    lui     t2, <hi20>
    addiw   t2, t2, <lo12>
    beq     t1, t2, .Ltmp0
    ebreak
  .Ltmp0:
    jarl    a0

Implement error handling code for the ebreak traps emitted for the
checks. This produces the following oops on a CFI failure (generated
using lkdtm):

[   21.177245] CFI failure at lkdtm_indirect_call+0x22/0x32 [lkdtm]
(target: lkdtm_increment_int+0x0/0x18 [lkdtm]; expected type: 0x3ad55aca)
[   21.178483] Kernel BUG [#1]
[   21.178671] Modules linked in: lkdtm
[   21.179037] CPU: 1 PID: 104 Comm: sh Not tainted
6.3.0-rc6-00037-g37d5ec6297ab #1
[   21.179511] Hardware name: riscv-virtio,qemu (DT)
[   21.179818] epc : lkdtm_indirect_call+0x22/0x32 [lkdtm]
[   21.180106]  ra : lkdtm_CFI_FORWARD_PROTO+0x48/0x7c [lkdtm]
[   21.180426] epc : ffffffff01387092 ra : ffffffff01386f14 sp : ff20000000453cf0
[   21.180792]  gp : ffffffff81308c38 tp : ff6000000243f080 t0 : ff20000000453b78
[   21.181157]  t1 : 000000003ad55aca t2 : 000000007e0c52a5 s0 : ff20000000453d00
[   21.181506]  s1 : 0000000000000001 a0 : ffffffff0138d170 a1 : ffffffff013870bc
[   21.181819]  a2 : b5fea48dd89aa700 a3 : 0000000000000001 a4 : 0000000000000fff
[   21.182169]  a5 : 0000000000000004 a6 : 00000000000000b7 a7 : 0000000000000000
[   21.182591]  s2 : ff20000000453e78 s3 : ffffffffffffffea s4 : 0000000000000012
[   21.183001]  s5 : ff600000023c7000 s6 : 0000000000000006 s7 : ffffffff013882a0
[   21.183653]  s8 : 0000000000000008 s9 : 0000000000000002 s10: ffffffff0138d878
[   21.184245]  s11: ffffffff0138d878 t3 : 0000000000000003 t4 : 0000000000000000
[   21.184591]  t5 : ffffffff8133df08 t6 : ffffffff8133df07
[   21.184858] status: 0000000000000120 badaddr: 0000000000000000
cause: 0000000000000003
[   21.185415] [<ffffffff01387092>] lkdtm_indirect_call+0x22/0x32 [lkdtm]
[   21.185772] [<ffffffff01386f14>] lkdtm_CFI_FORWARD_PROTO+0x48/0x7c [lkdtm]
[   21.186093] [<ffffffff01383552>] lkdtm_do_action+0x22/0x34 [lkdtm]
[   21.186445] [<ffffffff0138350c>] direct_entry+0x128/0x13a [lkdtm]
[   21.186817] [<ffffffff8033ed8c>] full_proxy_write+0x58/0xb2
[   21.187352] [<ffffffff801d4fe8>] vfs_write+0x14c/0x33a
[   21.187644] [<ffffffff801d5328>] ksys_write+0x64/0xd4
[   21.187832] [<ffffffff801d53a6>] sys_write+0xe/0x1a
[   21.188171] [<ffffffff80003996>] ret_from_syscall+0x0/0x2
[   21.188595] Code: 0513 0f65 a303 ffc5 53b7 7e0c 839b 2a53 0363 0073 (9002) 9582
[   21.189178] ---[ end trace 0000000000000000 ]---
[   21.189590] Kernel panic - not syncing: Fatal exception
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com> # ISA bits
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20230710183544.999540-12-samitolvanen@google.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>