Re: [PATCH v5] tpm: Prevent local DOS via tpm/tpm0/ppi/*operations

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Sep 23, 2025 at 01:07:48PM -0700, Nathan Chancellor wrote:
> Hi Denis,
> 
> On Mon, Sep 15, 2025 at 05:08:29PM -0400, Denis Aleksandrov wrote:
> > Reads on tpm/tpm0/ppi/*operations can become very long on
> > misconfigured systems. Reading the TPM is a blocking operation,
> > thus a user could effectively trigger a DOS.
> > 
> > Resolve this by caching the results and avoiding the blocking
> > operations after the first read.
> > 
> > Reported-by: Jan Stancek <jstancek@redhat.com>
> > Signed-off-by: Denis Aleksandrov <daleksan@redhat.com>
> > Suggested-by: Jarkko Sakkinen <jarkko@kernel.org>
> > Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
> > Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
> > ---
> > 
> > Changes in v5:
> > 	- Unlocks the tpm_ppi_lock if cache_ppi_operations() returns and
> > 	  error.
> > 
> >  drivers/char/tpm/tpm_ppi.c | 89 ++++++++++++++++++++++++++++----------
> >  1 file changed, 66 insertions(+), 23 deletions(-)
> > 
> > diff --git a/drivers/char/tpm/tpm_ppi.c b/drivers/char/tpm/tpm_ppi.c
> > index d53fce1c9d6f..47655407fea5 100644
> > --- a/drivers/char/tpm/tpm_ppi.c
> > +++ b/drivers/char/tpm/tpm_ppi.c
> > @@ -33,6 +33,20 @@ static const guid_t tpm_ppi_guid =
> >  	GUID_INIT(0x3DDDFAA6, 0x361B, 0x4EB4,
> >  		  0xA4, 0x24, 0x8D, 0x10, 0x08, 0x9D, 0x16, 0x53);
> >  
> > +static const char * const tpm_ppi_info[] = {
> > +	"Not implemented",
> > +	"BIOS only",
> > +	"Blocked for OS by system firmware",
> > +	"User required",
> > +	"User not required",
> > +};
> > +
> > +/* A spinlock to protect access to the cache from concurrent reads */
> > +static DEFINE_SPINLOCK(tpm_ppi_lock);
> > +
> > +static u32 ppi_operations_cache[PPI_VS_REQ_END + 1];
> > +static bool ppi_cache_populated;
> > +
> >  static bool tpm_ppi_req_has_parameter(u64 req)
> >  {
> >  	return req == 23;
> > @@ -277,8 +291,7 @@ static ssize_t tpm_show_ppi_response(struct device *dev,
> >  	return status;
> >  }
> >  
> > -static ssize_t show_ppi_operations(acpi_handle dev_handle, char *buf, u32 start,
> > -				   u32 end)
> > +static ssize_t cache_ppi_operations(acpi_handle dev_handle, char *buf)
> >  {
> >  	int i;
> >  	u32 ret;
> > @@ -286,34 +299,22 @@ static ssize_t show_ppi_operations(acpi_handle dev_handle, char *buf, u32 start,
> >  	union acpi_object *obj, tmp;
> >  	union acpi_object argv = ACPI_INIT_DSM_ARGV4(1, &tmp);
> >  
> > -	static char *info[] = {
> > -		"Not implemented",
> > -		"BIOS only",
> > -		"Blocked for OS by BIOS",
> > -		"User required",
> > -		"User not required",
> > -	};
> > -
> >  	if (!acpi_check_dsm(dev_handle, &tpm_ppi_guid, TPM_PPI_REVISION_ID_1,
> >  			    1 << TPM_PPI_FN_GETOPR))
> >  		return -EPERM;
> >  
> >  	tmp.integer.type = ACPI_TYPE_INTEGER;
> > -	for (i = start; i <= end; i++) {
> > +	for (i = 0; i <= PPI_VS_REQ_END; i++) {
> >  		tmp.integer.value = i;
> >  		obj = tpm_eval_dsm(dev_handle, TPM_PPI_FN_GETOPR,
> >  				   ACPI_TYPE_INTEGER, &argv,
> >  				   TPM_PPI_REVISION_ID_1);
> > -		if (!obj) {
> > +		if (!obj)
> >  			return -ENOMEM;
> > -		} else {
> > -			ret = obj->integer.value;
> > -			ACPI_FREE(obj);
> > -		}
> >  
> > -		if (ret > 0 && ret < ARRAY_SIZE(info))
> > -			len += sysfs_emit_at(buf, len, "%d %d: %s\n",
> > -					     i, ret, info[ret]);
> > +		ret = obj->integer.value;
> > +		ppi_operations_cache[i] = ret;
> > +		ACPI_FREE(obj);
> >  	}
> >  
> >  	return len;
> > @@ -324,9 +325,30 @@ static ssize_t tpm_show_ppi_tcg_operations(struct device *dev,
> >  					   char *buf)
> >  {
> >  	struct tpm_chip *chip = to_tpm_chip(dev);
> > +	ssize_t len = 0;
> > +	u32 ret;
> > +	int i;
> > +
> > +	spin_lock(&tpm_ppi_lock);
> > +	if (!ppi_cache_populated) {
> > +		len = cache_ppi_operations(chip->acpi_dev_handle, buf);
> > +		if (len < 0) {
> > +			spin_unlock(&tpm_ppi_lock);
> > +			return len;
> > +		}
> >  
> > -	return show_ppi_operations(chip->acpi_dev_handle, buf, 0,
> > -				   PPI_TPM_REQ_MAX);
> > +		ppi_cache_populated = true;
> > +	}
> > +
> > +	for (i = 0; i <= PPI_TPM_REQ_MAX; i++) {
> > +		ret = ppi_operations_cache[i];
> > +		if (ret >= 0 && ret < ARRAY_SIZE(tpm_ppi_info))
> > +			len += sysfs_emit_at(buf, len, "%d %d: %s\n",
> > +							i, ret, tpm_ppi_info[ret]);
> > +	}
> > +	spin_unlock(&tpm_ppi_lock);
> > +
> > +	return len;
> >  }
> >  
> >  static ssize_t tpm_show_ppi_vs_operations(struct device *dev,
> > @@ -334,9 +356,30 @@ static ssize_t tpm_show_ppi_vs_operations(struct device *dev,
> >  					  char *buf)
> >  {
> >  	struct tpm_chip *chip = to_tpm_chip(dev);
> > +	ssize_t len = 0;
> > +	u32 ret;
> > +	int i;
> >  
> > -	return show_ppi_operations(chip->acpi_dev_handle, buf, PPI_VS_REQ_START,
> > -				   PPI_VS_REQ_END);
> > +	spin_lock(&tpm_ppi_lock);
> > +	if (!ppi_cache_populated) {
> > +		len = cache_ppi_operations(chip->acpi_dev_handle, buf);
> > +		if (len < 0) {
> > +			spin_unlock(&tpm_ppi_lock);
> > +			return len;
> > +		}
> > +
> > +		ppi_cache_populated = true;
> > +	}
> > +
> > +	for (i = PPI_VS_REQ_START; i <= PPI_VS_REQ_END; i++) {
> > +		ret = ppi_operations_cache[i];
> > +		if (ret >= 0 && ret < ARRAY_SIZE(tpm_ppi_info))
> > +			len += sysfs_emit_at(buf, len, "%d %d: %s\n",
> > +							i, ret, tpm_ppi_info[ret]);
> > +	}
> > +	spin_unlock(&tpm_ppi_lock);
> > +
> > +	return len;
> >  }
> >  
> >  static DEVICE_ATTR(version, S_IRUGO, tpm_show_ppi_version, NULL);
> > -- 
> > 2.48.1
> > 
> 
> I am seeing a "scheduling while atomic" splat in -next when running
> LTP's read_all testcase against /proc and /sys that I bisected to this
> change (bisect log at the end of the message). It is still reproducible
> with the most recent sha in Jarkko's tree, c4a211c65878 ("tpm: Prevent
> local DOS via tpm/tpm0/ppi/*operations"), where there is no difference
> in the code as far as I can tell.
> 
>   $ curl -LSs https://github.com/nathanchance/env/raw/014a117384fb9121cf5c81ab30aa4de935246c17/bin/x86_64/read_all | install -m755 /dev/stdin read_all
> 
>   $ sudo sh -c "$PWD/read_all -d /proc && $PWD/read_all -d /sys && dmesg"
>   ...
>   [  103.605352] BUG: scheduling while atomic: read_all/2907/0x00000002
>   [  103.605357] Modules linked in: ...
>   [  103.605401]  ...
>   [  103.605454] CPU: 0 UID: 0 PID: 2907 Comm: read_all Not tainted 6.17.0-rc6-debug-00276-gc4a211c65878 #1 PREEMPT(full)  ccfbb8e489d66d107205aa22f3b6242dd3605b88
>   [  103.605457] Hardware name: AZW MINI S/MINI S, BIOS ADLNV106 05/12/2024
>   [  103.605459] Call Trace:
>   [  103.605461]  <TASK>
>   [  103.605465]  dump_stack_lvl+0x5d/0x80
>   [  103.605471]  __schedule_bug.cold+0x42/0x4e
>   [  103.605473]  __schedule+0x1083/0x1330
>   [  103.605478]  ? acpi_ex_field_datum_io+0xe8/0x4f0
>   [  103.605482]  ? acpi_os_release_object+0xe/0x20
>   [  103.605486]  schedule+0x27/0xd0
>   [  103.605487]  schedule_timeout+0xbd/0x100
>   [  103.605491]  __down_common+0x137/0x2d0
>   [  103.605493]  down_timeout+0x67/0x70
>   [  103.605495]  acpi_os_wait_semaphore+0x68/0x180
>   [  103.605498]  acpi_ut_acquire_mutex+0x97/0x250
>   [  103.605500]  acpi_ns_delete_namespace_subtree+0x48/0x110
>   [  103.605503]  acpi_ds_terminate_control_method+0x1c8/0x200
>   [  103.605505]  acpi_ps_parse_aml+0x1ae/0x5d0
>   [  103.605508]  acpi_ps_execute_method+0x171/0x3e0
>   [  103.605511]  acpi_ns_evaluate+0x196/0x5c0
>   [  103.605513]  acpi_evaluate_object+0x1ce/0x450
>   [  103.605515]  acpi_evaluate_dsm+0xcb/0x150
>   [  103.605519]  cache_ppi_operations.isra.0+0xc2/0x110
>   [  103.605522]  tpm_show_ppi_tcg_operations+0x99/0xb0
>   [  103.605523]  dev_attr_show+0x1c/0x50
>   [  103.605526]  sysfs_kf_seq_show+0xc9/0x120
>   [  103.605530]  seq_read_iter+0x125/0x480
>   [  103.605532]  ? rw_verify_area+0x56/0x180
>   [  103.605534]  vfs_read+0x265/0x390
>   [  103.605538]  ksys_read+0x73/0xf0
>   [  103.605540]  do_syscall_64+0x81/0x970
>   [  103.605542]  ? ksys_read+0x73/0xf0
>   [  103.605545]  ? refill_obj_stock+0x12e/0x240
>   [  103.605547]  ? xas_load+0xd/0xd0
>   [  103.605549]  ? xa_load+0x76/0xb0
>   [  103.605552]  ? refill_obj_stock+0x12e/0x240
>   [  103.605553]  ? __memcg_slab_free_hook+0xf4/0x140
>   [  103.605555]  ? kmem_cache_free+0x490/0x4d0
>   [  103.605557]  ? __x64_sys_close+0x3d/0x80
>   [  103.605560]  ? __x64_sys_close+0x3d/0x80
>   [  103.605562]  ? do_syscall_64+0x81/0x970
>   [  103.605563]  ? do_syscall_64+0x81/0x970
>   [  103.605564]  ? do_syscall_64+0x81/0x970
>   [  103.605565]  ? do_syscall_64+0x81/0x970
>   [  103.605566]  ? __irq_exit_rcu+0x4c/0xf0
>   [  103.605569]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>   [  103.605571] RIP: 0033:0x4243b8
>   [  103.605597] Code: 0f 05 48 83 f8 da 75 08 4c 89 c0 48 89 d6 0f 05 c3 48 89 f8 4d 89 c2 48 89 f7 4d 89 c8 48 89 d6 4c 8b 4c 24 08 48 89 ca 0f 05 <c3> e9 e1 ff ff ff 48 8d 3d 9b 52 02 00 e9 8a 06 00 00 48 8d 3d 8f
>   [  103.605598] RSP: 002b:00007ffccef321b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
>   [  103.605601] RAX: ffffffffffffffda RBX: 00007ffccef32690 RCX: 00000000004243b8
>   [  103.605602] RDX: 00000000000003ff RSI: 00007ffccef32690 RDI: 0000000000000003
>   [  103.605603] RBP: 00000000310cdd71 R08: 0000000000000000 R09: 0000000000000000
>   [  103.605603] R10: 0000000000000000 R11: 0000000000000246 R12: 00007fb294164000
>   [  103.605604] R13: 000000000042b00c R14: 00007ffccef32290 R15: 0000000000000003
>   [  103.605606]  </TASK>
>   [  103.652735] BUG: scheduling while atomic: read_all/2907/0x00000000
>   [  103.652739] Modules linked in: ...
>   [  103.652775]  ...
>   [  103.652825] CPU: 0 UID: 0 PID: 2907 Comm: read_all Tainted: G        W           6.17.0-rc6-debug-00276-gc4a211c65878 #1 PREEMPT(full)  ccfbb8e489d66d107205aa22f3b6242dd3605b88
>   [  103.652828] Tainted: [W]=WARN
>   [  103.652829] Hardware name: AZW MINI S/MINI S, BIOS ADLNV106 05/12/2024
>   [  103.652830] Call Trace:
>   [  103.652830]  <TASK>
>   [  103.652831]  dump_stack_lvl+0x5d/0x80
>   [  103.652835]  __schedule_bug.cold+0x42/0x4e
>   [  103.652837]  __schedule+0x1083/0x1330
>   [  103.652840]  ? get_nohz_timer_target+0x2f/0x150
>   [  103.652843]  ? timerqueue_add+0x73/0xd0
>   [  103.652845]  schedule+0x27/0xd0
>   [  103.652847]  schedule_hrtimeout_range_clock+0xd8/0x120
>   [  103.652850]  ? __pfx_hrtimer_wakeup+0x10/0x10
>   [  103.652853]  usleep_range_state+0x6c/0xa0
>   [  103.652855]  crb_wait_for_reg_32.constprop.0+0x40/0x80
>   [  103.652858]  crb_request_locality+0x3d/0x50
>   [  103.652860]  tpm_chip_start+0x6c/0xe0
>   [  103.652862]  tpm_try_get_ops+0x89/0xb0
>   [  103.652863]  tpm_find_get_ops+0x1b/0x70
>   [  103.652865]  tpm_pcr_read+0x1b/0x70
>   [  103.652866]  pcr_value_show+0xcc/0x140
>   [  103.652869]  dev_attr_show+0x1c/0x50
>   [  103.652871]  sysfs_kf_seq_show+0xc9/0x120
>   [  103.652873]  seq_read_iter+0x125/0x480
>   [  103.652875]  ? rw_verify_area+0x56/0x180
>   [  103.652877]  vfs_read+0x265/0x390
>   [  103.652880]  ksys_read+0x73/0xf0
>   [  103.652882]  do_syscall_64+0x81/0x970
>   [  103.652883]  ? do_syscall_64+0x81/0x970
>   [  103.652884]  ? __irq_exit_rcu+0x4c/0xf0
>   [  103.652887]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>   [  103.652889] RIP: 0033:0x4243b8
>   [  103.652903] Code: 0f 05 48 83 f8 da 75 08 4c 89 c0 48 89 d6 0f 05 c3 48 89 f8 4d 89 c2 48 89 f7 4d 89 c8 48 89 d6 4c 8b 4c 24 08 48 89 ca 0f 05 <c3> e9 e1 ff ff ff 48 8d 3d 9b 52 02 00 e9 8a 06 00 00 48 8d 3d 8f
>   [  103.652905] RSP: 002b:00007ffccef321b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
>   [  103.652906] RAX: ffffffffffffffda RBX: 00007ffccef32690 RCX: 00000000004243b8
>   [  103.652907] RDX: 00000000000003ff RSI: 00007ffccef32690 RDI: 0000000000000003
>   [  103.652908] RBP: 00000000310cdd71 R08: 0000000000000000 R09: 0000000000000000
>   [  103.652909] R10: 0000000000000000 R11: 0000000000000246 R12: 00007fb294164000
>   [  103.652910] R13: 000000000042b00c R14: 00007ffccef32290 R15: 0000000000000003
>   [  103.652912]  </TASK>
> 
> If there is any other information I can provide or patches I can test, I
> am more than happy to do so.

Thanks a lot! And I have not rushed with my 6.18 pull request.

It took me less than 30 seconds to locate the bug: it's spin
lock and sleeing operations.

E.g., acpi_evaluate_dsm_typed can lead to kzalloc() and stuff
like that. I don't know how I could I have possibly missed this
detail and this is embarrasing but luckily this should be easy
to fix with major hurdle :-)

What I suggest is that I'll simply repeal and replace the lock
type (i.e. tweak the patch), as it does not feel worth of trouble
to do a review round. Then we should be seeing better results.

Thanks again for spotting this. Yeah, and definitely not blaming
original author for this. It's all on me tbh. The patch itself
was great and I should have been able to address this...

> 
> Cheers,
> Nathan

BR, Jarkko