Re: [PATCH v2] powerpc/rtas: Keep MSR[RI] set when calling RTAS

From: Michael Ellerman
Date: Tue May 03 2022 - 11:06:51 EST

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Laurent Dufour <ldufour@xxxxxxxxxxxxx> 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).

> 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@xxxxxxxxxxxxxxx
> Suggested-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
> ---
> 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.

> __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"

> + * 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?

cheers

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