Re: [PATCH 1/3] tpm: efi: Use local variable for calculating final log size

[Jarkko Sakkinen]

On Mon, Mar 08, 2021 at 10:19:52PM -0500, Stefan Berger wrote:
> When tpm_read_log_efi was called multiple times, which happens when one
> loads and unloads a TPM2 driver multiple times, then the global variable
> efi_tpm_final_log_size will at some point become a negative number due
> to the subtraction of final_events_preboot_size occurring each time. Use
> a local_efi_tpm_final_log_size to avoid this integer underflow.
> 
> The following issue is now resolved:
> 
> Mar  8 15:35:12 hibinst kernel: Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
> Mar  8 15:35:12 hibinst kernel: Workqueue: tpm-vtpm vtpm_proxy_work [tpm_vtpm_proxy]
> Mar  8 15:35:12 hibinst kernel: RIP: 0010:__memcpy+0x12/0x20
> Mar  8 15:35:12 hibinst kernel: Code: 00 b8 01 00 00 00 85 d2 74 0a c7 05 44 7b ef 00 0f 00 00 00 c3 cc cc cc 66 66 90 66 90 48 89 f8 48 89 d1 48 c1 e9 03 83 e2 07 <f3> 48 a5 89 d1 f3 a4 c3 66 0f 1f 44 00 00 48 89 f8 48 89 d1 f3 a4
> Mar  8 15:35:12 hibinst kernel: RSP: 0018:ffff9ac4c0fcfde0 EFLAGS: 00010206
> Mar  8 15:35:12 hibinst kernel: RAX: ffff88f878cefed5 RBX: ffff88f878ce9000 RCX: 1ffffffffffffe0f
> Mar  8 15:35:12 hibinst kernel: RDX: 0000000000000003 RSI: ffff9ac4c003bff9 RDI: ffff88f878cf0e4d
> Mar  8 15:35:12 hibinst kernel: RBP: ffff9ac4c003b000 R08: 0000000000001000 R09: 000000007e9d6073
> Mar  8 15:35:12 hibinst kernel: R10: ffff9ac4c003b000 R11: ffff88f879ad3500 R12: 0000000000000ed5
> Mar  8 15:35:12 hibinst kernel: R13: ffff88f878ce9760 R14: 0000000000000002 R15: ffff88f77de7f018
> Mar  8 15:35:12 hibinst kernel: FS:  0000000000000000(0000) GS:ffff88f87bd00000(0000) knlGS:0000000000000000
> Mar  8 15:35:12 hibinst kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> Mar  8 15:35:12 hibinst kernel: CR2: ffff9ac4c003c000 CR3: 00000001785a6004 CR4: 0000000000060ee0
> Mar  8 15:35:12 hibinst kernel: Call Trace:
> Mar  8 15:35:12 hibinst kernel: tpm_read_log_efi+0x152/0x1a7
> Mar  8 15:35:12 hibinst kernel: tpm_bios_log_setup+0xc8/0x1c0
> Mar  8 15:35:12 hibinst kernel: tpm_chip_register+0x8f/0x260
> Mar  8 15:35:12 hibinst kernel: vtpm_proxy_work+0x16/0x60 [tpm_vtpm_proxy]
> Mar  8 15:35:12 hibinst kernel: process_one_work+0x1b4/0x370
> Mar  8 15:35:12 hibinst kernel: worker_thread+0x53/0x3e0
> Mar  8 15:35:12 hibinst kernel: ? process_one_work+0x370/0x370
> 
> Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> ---
>  drivers/char/tpm/eventlog/efi.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/char/tpm/eventlog/efi.c b/drivers/char/tpm/eventlog/efi.c
> index 35229e5143ca..b6ffb5faf416 100644
> --- a/drivers/char/tpm/eventlog/efi.c
> +++ b/drivers/char/tpm/eventlog/efi.c
> @@ -18,6 +18,7 @@ int tpm_read_log_efi(struct tpm_chip *chip)
>  
>  	struct efi_tcg2_final_events_table *final_tbl = NULL;
>  	struct linux_efi_tpm_eventlog *log_tbl;
> +	int local_efi_tpm_final_log_size;
>  	struct tpm_bios_log *log;
>  	u32 log_size;
>  	u8 tpm_log_version;
> @@ -80,10 +81,11 @@ int tpm_read_log_efi(struct tpm_chip *chip)
>  		goto out;
>  	}
>  
> -	efi_tpm_final_log_size -= log_tbl->final_events_preboot_size;
> +	local_efi_tpm_final_log_size = efi_tpm_final_log_size -
> +					log_tbl->final_events_preboot_size;

This starts to have so many weird locals that an inline comment here
in plain Enlighs would be nice explaining the calculation.

>  
>  	tmp = krealloc(log->bios_event_log,
> -		       log_size + efi_tpm_final_log_size,
> +		       log_size + local_efi_tpm_final_log_size,

Ditto.

>  		       GFP_KERNEL);
>  	if (!tmp) {
>  		kfree(log->bios_event_log);
> @@ -100,9 +102,9 @@ int tpm_read_log_efi(struct tpm_chip *chip)
>  	 */
>  	memcpy((void *)log->bios_event_log + log_size,
>  	       final_tbl->events + log_tbl->final_events_preboot_size,
> -	       efi_tpm_final_log_size);
> +	       local_efi_tpm_final_log_size);
>  	log->bios_event_log_end = log->bios_event_log +
> -		log_size + efi_tpm_final_log_size;
> +		log_size + local_efi_tpm_final_log_size;

Ditto.

>  
>  out:
>  	memunmap(final_tbl);
> -- 
> 2.29.2
> 
> 

I think this is good chance to improve the documentation a bit.

/Jarkko