linux-kernel - Re: [PATCH 4/6] riscv: Add CFI error handling

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Message-ID: <202306301126.D7EC815A22@keescook>
Date:   Fri, 30 Jun 2023 11:26:59 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-riscv@...ts.infradead.org, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] riscv: Add CFI error handling

On Thu, Jun 29, 2023 at 11:42:49PM +0000, Sami Tolvanen wrote:
> 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
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

Looks good -- should the noaddr failure paths include any warnings of
their own? (i.e. isn't that unexpected?)

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook