Re: [PATCH v2] powerpc/rtas_flash: allow user copy to flash block cache objects

[Kees Cook]

On Thu, Aug 10, 2023 at 10:37:55PM -0500, Nathan Lynch via B4 Relay wrote:
> From: Nathan Lynch <nathanl@xxxxxxxxxxxxx>
> 
> With hardened usercopy enabled (CONFIG_HARDENED_USERCOPY=y), using the
> /proc/powerpc/rtas/firmware_update interface to prepare a system
> firmware update yields a BUG():
> 
> kernel BUG at mm/usercopy.c:102!
> Oops: Exception in kernel mode, sig: 5 [#1]
> LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
> Modules linked in:
> CPU: 0 PID: 2232 Comm: dd Not tainted 6.5.0-rc3+ #2
> Hardware name: IBM,8408-E8E POWER8E (raw) 0x4b0201 0xf000004 of:IBM,FW860.50 (SV860_146) hv:phyp pSeries
> NIP:  c0000000005991d0 LR: c0000000005991cc CTR: 0000000000000000
> REGS: c0000000148c76a0 TRAP: 0700   Not tainted  (6.5.0-rc3+)
> MSR:  8000000000029033 <SF,EE,ME,IR,DR,RI,LE>  CR: 24002242  XER: 0000000c
> CFAR: c0000000001fbd34 IRQMASK: 0
> [ ... GPRs omitted ... ]
> NIP [c0000000005991d0] usercopy_abort+0xa0/0xb0
> LR [c0000000005991cc] usercopy_abort+0x9c/0xb0
> Call Trace:
> [c0000000148c7940] [c0000000005991cc] usercopy_abort+0x9c/0xb0 (unreliable)
> [c0000000148c79b0] [c000000000536814] __check_heap_object+0x1b4/0x1d0
> [c0000000148c79f0] [c000000000599080] __check_object_size+0x2d0/0x380
> [c0000000148c7a30] [c000000000045ed4] rtas_flash_write+0xe4/0x250
> [c0000000148c7a80] [c00000000068a0fc] proc_reg_write+0xfc/0x160
> [c0000000148c7ab0] [c0000000005a381c] vfs_write+0xfc/0x4e0
> [c0000000148c7b70] [c0000000005a3e10] ksys_write+0x90/0x160
> [c0000000148c7bc0] [c00000000002f2c8] system_call_exception+0x178/0x320
> [c0000000148c7e50] [c00000000000d520] system_call_common+0x160/0x2c4
> --- interrupt: c00 at 0x7fff9f17e5e4
> 
> The blocks of the firmware image are copied directly from user memory
> to objects allocated from flash_block_cache, so flash_block_cache must
> be created using kmem_cache_create_usercopy() to mark it safe for user
> access.
> 
> Fixes: 6d07d1cd300f ("usercopy: Restrict non-usercopy caches to size 0")
> Signed-off-by: Nathan Lynch <nathanl@xxxxxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook