Date:   Tue, 3 May 2022 18:23:28 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        stable@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2] powerpc/rtas: Keep MSR[RI] set when calling RTAS

Thanks Michael for reviewing this.

On 03/05/2022, 17:06:41, Michael Ellerman wrote:
> Laurent Dufour <ldufour@...ux.ibm.com> writes:
>> RTAS runs in real mode (MSR[DR] and MSR[IR] unset) and in 32bits
>> mode (MSR[SF] unset).
> 
> Probably also worth mentioning that it runs in big endian mode :)
> 
> It is specified in PAPR (R1-7.2.1-6).

Sure!

> 
>> The change in MSR is done in enter_rtas() in a relatively complex way,
>> since the MSR value could be hardcoded.
>>
>> Furthermore, a panic has been reported when hitting the watchdog interrupt
>> while running in RTAS, this leads to the following stack trace:
>>
>> [69244.027433][   C24] watchdog: CPU 24 Hard LOCKUP
>> [69244.027442][   C24] watchdog: CPU 24 TB:997512652051031, last heartbeat TB:997504470175378 (15980ms ago)
>> [69244.027451][   C24] Modules linked in: chacha_generic(E) libchacha(E) xxhash_generic(E) wp512(E) sha3_generic(E) rmd160(E) poly1305_generic(E) libpoly1305(E) michael_mic(E) md4(E) crc32_generic(E) cmac(E) ccm(E) algif_rng(E) twofish_generic(E) twofish_common(E) serpent_generic(E) fcrypt(E) des_generic(E) libdes(E) cast6_generic(E) cast5_generic(E) cast_common(E) camellia_generic(E) blowfish_generic(E) blowfish_common(E) algif_skcipher(E) algif_hash(E) gcm(E) algif_aead(E) af_alg(E) tun(E) rpcsec_gss_krb5(E) auth_rpcgss(E)
>> nfsv4(E) dns_resolver(E) rpadlpar_io(EX) rpaphp(EX) xsk_diag(E) tcp_diag(E) udp_diag(E) raw_diag(E) inet_diag(E) unix_diag(E) af_packet_diag(E) netlink_diag(E) nfsv3(E) nfs_acl(E) nfs(E) lockd(E) grace(E) sunrpc(E) fscache(E) netfs(E) af_packet(E) rfkill(E) bonding(E) tls(E) ibmveth(EX) crct10dif_vpmsum(E) rtc_generic(E) drm(E) drm_panel_orientation_quirks(E) fuse(E) configfs(E) backlight(E) ip_tables(E) x_tables(E) dm_service_time(E) sd_mod(E) t10_pi(E)
>> [69244.027555][   C24]  ibmvfc(EX) scsi_transport_fc(E) vmx_crypto(E) gf128mul(E) btrfs(E) blake2b_generic(E) libcrc32c(E) crc32c_vpmsum(E) xor(E) raid6_pq(E) dm_mirror(E) dm_region_hash(E) dm_log(E) sg(E) dm_multipath(E) dm_mod(E) scsi_dh_rdac(E) scsi_dh_emc(E) scsi_dh_alua(E) scsi_mod(E)
>> [69244.027587][   C24] Supported: No, Unreleased kernel
>> [69244.027600][   C24] CPU: 24 PID: 87504 Comm: drmgr Kdump: loaded Tainted: G            E  X    5.14.21-150400.71.1.bz196362_2-default #1 SLE15-SP4 (unreleased) 0d821077ef4faa8dfaf370efb5fdca1fa35f4e2c
>> [69244.027609][   C24] NIP:  000000001fb41050 LR: 000000001fb4104c CTR: 0000000000000000
>> [69244.027612][   C24] REGS: c00000000fc33d60 TRAP: 0100   Tainted: G            E  X     (5.14.21-150400.71.1.bz196362_2-default)
>> [69244.027615][   C24] MSR:  8000000002981000 <SF,VEC,VSX,ME>  CR: 48800002  XER: 20040020
>> [69244.027625][   C24] CFAR: 000000000000011c IRQMASK: 1
>> [69244.027625][   C24] GPR00: 0000000000000003 ffffffffffffffff 0000000000000001 00000000000050dc
>> [69244.027625][   C24] GPR04: 000000001ffb6100 0000000000000020 0000000000000001 000000001fb09010
>> [69244.027625][   C24] GPR08: 0000000020000000 0000000000000000 0000000000000000 0000000000000000
>> [69244.027625][   C24] GPR12: 80040000072a40a8 c00000000ff8b680 0000000000000007 0000000000000034
>> [69244.027625][   C24] GPR16: 000000001fbf6e94 000000001fbf6d84 000000001fbd1db0 000000001fb3f008
>> [69244.027625][   C24] GPR20: 000000001fb41018 ffffffffffffffff 000000000000017f fffffffffffff68f
>> [69244.027625][   C24] GPR24: 000000001fb18fe8 000000001fb3e000 000000001fb1adc0 000000001fb1cf40
>> [69244.027625][   C24] GPR28: 000000001fb26000 000000001fb460f0 000000001fb17f18 000000001fb17000
>> [69244.027663][   C24] NIP [000000001fb41050] 0x1fb41050
>> [69244.027696][   C24] LR [000000001fb4104c] 0x1fb4104c
>> [69244.027699][   C24] Call Trace:
>> [69244.027701][   C24] Instruction dump:
>> [69244.027723][   C24] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
>> [69244.027728][   C24] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
>> [69244.027762][T87504] Oops: Unrecoverable System Reset, sig: 6 [#1]
>> [69244.028044][T87504] LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
>> [69244.028089][T87504] Modules linked in: chacha_generic(E) libchacha(E) xxhash_generic(E) wp512(E) sha3_generic(E) rmd160(E) poly1305_generic(E) libpoly1305(E) michael_mic(E) md4(E) crc32_generic(E) cmac(E) ccm(E) algif_rng(E) twofish_generic(E) twofish_common(E) serpent_generic(E) fcrypt(E) des_generic(E) libdes(E) cast6_generic(E) cast5_generic(E) cast_common(E) camellia_generic(E) blowfish_generic(E) blowfish_common(E) algif_skcipher(E) algif_hash(E) gcm(E) algif_aead(E) af_alg(E) tun(E) rpcsec_gss_krb5(E) auth_rpcgss(E)
>> nfsv4(E) dns_resolver(E) rpadlpar_io(EX) rpaphp(EX) xsk_diag(E) tcp_diag(E) udp_diag(E) raw_diag(E) inet_diag(E) unix_diag(E) af_packet_diag(E) netlink_diag(E) nfsv3(E) nfs_acl(E) nfs(E) lockd(E) grace(E) sunrpc(E) fscache(E) netfs(E) af_packet(E) rfkill(E) bonding(E) tls(E) ibmveth(EX) crct10dif_vpmsum(E) rtc_generic(E) drm(E) drm_panel_orientation_quirks(E) fuse(E) configfs(E) backlight(E) ip_tables(E) x_tables(E) dm_service_time(E) sd_mod(E) t10_pi(E)
>> [69244.028171][T87504]  ibmvfc(EX) scsi_transport_fc(E) vmx_crypto(E) gf128mul(E) btrfs(E) blake2b_generic(E) libcrc32c(E) crc32c_vpmsum(E) xor(E) raid6_pq(E) dm_mirror(E) dm_region_hash(E) dm_log(E) sg(E) dm_multipath(E) dm_mod(E) scsi_dh_rdac(E) scsi_dh_emc(E) scsi_dh_alua(E) scsi_mod(E)
>> [69244.028307][T87504] Supported: No, Unreleased kernel
>> [69244.028385][T87504] CPU: 24 PID: 87504 Comm: drmgr Kdump: loaded Tainted: G            E  X    5.14.21-150400.71.1.bz196362_2-default #1 SLE15-SP4 (unreleased) 0d821077ef4faa8dfaf370efb5fdca1fa35f4e2c
>> [69244.028408][T87504] NIP:  000000001fb41050 LR: 000000001fb4104c CTR: 0000000000000000
>> [69244.028418][T87504] REGS: c00000000fc33d60 TRAP: 0100   Tainted: G            E  X     (5.14.21-150400.71.1.bz196362_2-default)
>> [69244.028429][T87504] MSR:  8000000002981000 <SF,VEC,VSX,ME>  CR: 48800002  XER: 20040020
>> [69244.028444][T87504] CFAR: 000000000000011c IRQMASK: 1
>> [69244.028444][T87504] GPR00: 0000000000000003 ffffffffffffffff 0000000000000001 00000000000050dc
>> [69244.028444][T87504] GPR04: 000000001ffb6100 0000000000000020 0000000000000001 000000001fb09010
>> [69244.028444][T87504] GPR08: 0000000020000000 0000000000000000 0000000000000000 0000000000000000
>> [69244.028444][T87504] GPR12: 80040000072a40a8 c00000000ff8b680 0000000000000007 0000000000000034
>> [69244.028444][T87504] GPR16: 000000001fbf6e94 000000001fbf6d84 000000001fbd1db0 000000001fb3f008
>> [69244.028444][T87504] GPR20: 000000001fb41018 ffffffffffffffff 000000000000017f fffffffffffff68f
>> [69244.028444][T87504] GPR24: 000000001fb18fe8 000000001fb3e000 000000001fb1adc0 000000001fb1cf40
>> [69244.028444][T87504] GPR28: 000000001fb26000 000000001fb460f0 000000001fb17f18 000000001fb17000
>> [69244.028534][T87504] NIP [000000001fb41050] 0x1fb41050
>> [69244.028543][T87504] LR [000000001fb4104c] 0x1fb4104c
>> [69244.028549][T87504] Call Trace:
>> [69244.028554][T87504] Instruction dump:
>> [69244.028561][T87504] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
>> [69244.028575][T87504] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
>> [69244.028607][T87504] ---[ end trace 3ddec07f638c34a2 ]---
>>
>> This happens because MSR[RI] is unset when entering RTAS but there is no
>> valid reason to not set it here.
>>
>> RTAS is expected to be called with MSR[RI] as specified in PAPR+ section
>> "7.2.1 Machine State":
>>
>>  R1–7.2.1–9. If called with MSR[RI] equal to 1, then RTAS must protect its
>>  own critical regions from recursion by setting the MSRRI bit to 0 when in
>>  the critical regions.
>>
>> Fixing this by reviewing the way MSR is compute before calling RTAS. Now a
>> hardcoded value meaning real mode, 32 bits and Recoverable Interrupt is
>> loaded.
>>
>> In addition a check is added in do_enter_rtas() to detect calls made with
>> MSR[RI] unset, as we are forcing it on later.
>>
>> This patch has been tested on the following machines:
>> Power KVM Guest
>>   P8 S822L (host Ubuntu kernel 5.11.0-49-generic)
>> PowerVM LPAR
>>   P8 9119-MME (FW860.A1)
>>   p9 9008-22L (FW950.00)
>>   P10 9080-HEX (FW1010.00)
>>
>> Changes in V2:
>>  - Change comment in code to indicate NMI (Nick's comment)
>>  - Add reference to PAPR+ in the change log (Michael's comment)
>>
>> Cc: stable@...r.kernel.org
>> Suggested-by: Nicholas Piggin <npiggin@...il.com>
>> Signed-off-by: Laurent Dufour <ldufour@...ux.ibm.com>
>> ---
>>  arch/powerpc/kernel/entry_64.S | 20 ++++++++------------
>>  arch/powerpc/kernel/rtas.c     |  5 +++++
>>  2 files changed, 13 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
>> index 9581906b5ee9..65cb14b56f8d 100644
>> --- a/arch/powerpc/kernel/entry_64.S
>> +++ b/arch/powerpc/kernel/entry_64.S
>> @@ -330,22 +330,18 @@ _GLOBAL(enter_rtas)
>>  	clrldi	r4,r4,2			/* convert to realmode address */
>>         	mtlr	r4
>>  
>> -	li	r0,0
>> -	ori	r0,r0,MSR_EE|MSR_SE|MSR_BE|MSR_RI
>> -	andc	r0,r6,r0
>> -	
>> -        li      r9,1
>> -        rldicr  r9,r9,MSR_SF_LG,(63-MSR_SF_LG)
>> -	ori	r9,r9,MSR_IR|MSR_DR|MSR_FE0|MSR_FE1|MSR_FP|MSR_RI|MSR_LE
>> -	andc	r6,r0,r9
>  
> One advantage of the old method is it can adapt to new MSR bits being
> set by the kernel.
> 
> For example we used to use RTAS on powernv, and this code didn't need
> updating to cater to MSR_HV being set. We will probably never use RTAS
> on bare-metal again, so that's OK.
> 
> But your change might break secure virtual machines, because it clears
> MSR_S whereas the old code didn't. I think SVMs did use RTAS, but I
> don't know whether it matters if it's called with MSR_S set or not?
> 
> Not sure if anyone will remember, or has a working setup they can test.
> Maybe for now we just copy MSR_S from the kernel MSR the way the
> current code does.

I could update the code to deal with MSR[S], but I can't see how I would
test that :/

> 
>>  __enter_rtas:
>> -	sync				/* disable interrupts so SRR0/1 */
>> -	mtmsrd	r0			/* don't get trashed */
>> -
>>  	LOAD_REG_ADDR(r4, rtas)
>>  	ld	r5,RTASENTRY(r4)	/* get the rtas->entry value */
>>  	ld	r4,RTASBASE(r4)		/* get the rtas->base value */
>> +
>> +	/* RTAS runs in 32bits real mode but let MSR[]RI on as we may hit
> 
> "32-bit big endian real mode"

Yep!

> 
>> +	 * NMI (SRESET or MCE). RTAS should disable RI in its critical
>> +	 * regions (as specified in PAPR+ section 7.2.1). */
>> +	LOAD_REG_IMMEDIATE(r6, MSR_ME|MSR_RI)
>> +
>> +	li      r0,0
>> +	mtmsrd  r0,1                    /* disable RI before using SRR0/1 */
>>  	
>>  	mtspr	SPRN_SRR0,r5
>>  	mtspr	SPRN_SRR1,r6
>> diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
>> index 1f42aabbbab3..d7775b8c8853 100644
>> --- a/arch/powerpc/kernel/rtas.c
>> +++ b/arch/powerpc/kernel/rtas.c
>> @@ -49,6 +49,11 @@ void enter_rtas(unsigned long);
>>  
>>  static inline void do_enter_rtas(unsigned long args)
>>  {
>> +	unsigned long msr;
>> +
>> +	msr = mfmsr();
>> +	BUG_ON(!(msr & MSR_RI));
> 
> I'm not sure about this.
> 
> We call RTAS in some low-level places, so if we ever hit this BUG_ON
> then it might cause us to crash badly, or recursively BUG.
> 
> A WARN_ON_ONCE() might be safer?

I'm afraid a BUG_ON is required here. Since MSR[RI] is set on RTAS exit so
if it was not set when calling RTAS, that's a real issue and should
generate unexpected behaviour.

Do you have places in mind where RTAS could be called with !MSR[RI]?

Cheers,
Laurent.

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