This is a note to let you know that I've just added the patch titled s390/archrandom: prevent CPACF trng invocations in interrupt context to the 5.18-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: s390-archrandom-prevent-cpacf-trng-invocations-in-interrupt-context.patch and it can be found in the queue-5.18 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 918e75f77af7d2e049bb70469ec0a2c12782d96a Mon Sep 17 00:00:00 2001 From: Harald Freudenberger <freude@xxxxxxxxxxxxx> Date: Wed, 13 Jul 2022 15:17:21 +0200 Subject: s390/archrandom: prevent CPACF trng invocations in interrupt context From: Harald Freudenberger <freude@xxxxxxxxxxxxx> commit 918e75f77af7d2e049bb70469ec0a2c12782d96a upstream. This patch slightly reworks the s390 arch_get_random_seed_{int,long} implementation: Make sure the CPACF trng instruction is never called in any interrupt context. This is done by adding an additional condition in_task(). Justification: There are some constrains to satisfy for the invocation of the arch_get_random_seed_{int,long}() functions: - They should provide good random data during kernel initialization. - They should not be called in interrupt context as the TRNG instruction is relatively heavy weight and may for example make some network loads cause to timeout and buck. However, it was not clear what kind of interrupt context is exactly encountered during kernel init or network traffic eventually calling arch_get_random_seed_long(). After some days of investigations it is clear that the s390 start_kernel function is not running in any interrupt context and so the trng is called: Jul 11 18:33:39 t35lp54 kernel: [<00000001064e90ca>] arch_get_random_seed_long.part.0+0x32/0x70 Jul 11 18:33:39 t35lp54 kernel: [<000000010715f246>] random_init+0xf6/0x238 Jul 11 18:33:39 t35lp54 kernel: [<000000010712545c>] start_kernel+0x4a4/0x628 Jul 11 18:33:39 t35lp54 kernel: [<000000010590402a>] startup_continue+0x2a/0x40 The condition in_task() is true and the CPACF trng provides random data during kernel startup. The network traffic however, is more difficult. A typical call stack looks like this: Jul 06 17:37:07 t35lp54 kernel: [<000000008b5600fc>] extract_entropy.constprop.0+0x23c/0x240 Jul 06 17:37:07 t35lp54 kernel: [<000000008b560136>] crng_reseed+0x36/0xd8 Jul 06 17:37:07 t35lp54 kernel: [<000000008b5604b8>] crng_make_state+0x78/0x340 Jul 06 17:37:07 t35lp54 kernel: [<000000008b5607e0>] _get_random_bytes+0x60/0xf8 Jul 06 17:37:07 t35lp54 kernel: [<000000008b56108a>] get_random_u32+0xda/0x248 Jul 06 17:37:07 t35lp54 kernel: [<000000008aefe7a8>] kfence_guarded_alloc+0x48/0x4b8 Jul 06 17:37:07 t35lp54 kernel: [<000000008aeff35e>] __kfence_alloc+0x18e/0x1b8 Jul 06 17:37:07 t35lp54 kernel: [<000000008aef7f10>] __kmalloc_node_track_caller+0x368/0x4d8 Jul 06 17:37:07 t35lp54 kernel: [<000000008b611eac>] kmalloc_reserve+0x44/0xa0 Jul 06 17:37:07 t35lp54 kernel: [<000000008b611f98>] __alloc_skb+0x90/0x178 Jul 06 17:37:07 t35lp54 kernel: [<000000008b6120dc>] __napi_alloc_skb+0x5c/0x118 Jul 06 17:37:07 t35lp54 kernel: [<000000008b8f06b4>] qeth_extract_skb+0x13c/0x680 Jul 06 17:37:07 t35lp54 kernel: [<000000008b8f6526>] qeth_poll+0x256/0x3f8 Jul 06 17:37:07 t35lp54 kernel: [<000000008b63d76e>] __napi_poll.constprop.0+0x46/0x2f8 Jul 06 17:37:07 t35lp54 kernel: [<000000008b63dbec>] net_rx_action+0x1cc/0x408 Jul 06 17:37:07 t35lp54 kernel: [<000000008b937302>] __do_softirq+0x132/0x6b0 Jul 06 17:37:07 t35lp54 kernel: [<000000008abf46ce>] __irq_exit_rcu+0x13e/0x170 Jul 06 17:37:07 t35lp54 kernel: [<000000008abf531a>] irq_exit_rcu+0x22/0x50 Jul 06 17:37:07 t35lp54 kernel: [<000000008b922506>] do_io_irq+0xe6/0x198 Jul 06 17:37:07 t35lp54 kernel: [<000000008b935826>] io_int_handler+0xd6/0x110 Jul 06 17:37:07 t35lp54 kernel: [<000000008b9358a6>] psw_idle_exit+0x0/0xa Jul 06 17:37:07 t35lp54 kernel: ([<000000008ab9c59a>] arch_cpu_idle+0x52/0xe0) Jul 06 17:37:07 t35lp54 kernel: [<000000008b933cfe>] default_idle_call+0x6e/0xd0 Jul 06 17:37:07 t35lp54 kernel: [<000000008ac59f4e>] do_idle+0xf6/0x1b0 Jul 06 17:37:07 t35lp54 kernel: [<000000008ac5a28e>] cpu_startup_entry+0x36/0x40 Jul 06 17:37:07 t35lp54 kernel: [<000000008abb0d90>] smp_start_secondary+0x148/0x158 Jul 06 17:37:07 t35lp54 kernel: [<000000008b935b9e>] restart_int_handler+0x6e/0x90 which confirms that the call is in softirq context. So in_task() covers exactly the cases where we want to have CPACF trng called: not in nmi, not in hard irq, not in soft irq but in normal task context and during kernel init. Signed-off-by: Harald Freudenberger <freude@xxxxxxxxxxxxx> Acked-by: Jason A. Donenfeld <Jason@xxxxxxxxx> Reviewed-by: Juergen Christ <jchrist@xxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20220713131721.257907-1-freude@xxxxxxxxxxxxx Fixes: e4f74400308c ("s390/archrandom: simplify back to earlier design and initialize earlier") [agordeev@xxxxxxxxxxxxx changed desc, added Fixes and Link, removed -stable] Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/s390/include/asm/archrandom.h | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) --- a/arch/s390/include/asm/archrandom.h +++ b/arch/s390/include/asm/archrandom.h @@ -2,7 +2,7 @@ /* * Kernel interface for the s390 arch_random_* functions * - * Copyright IBM Corp. 2017, 2020 + * Copyright IBM Corp. 2017, 2022 * * Author: Harald Freudenberger <freude@xxxxxxxxxx> * @@ -14,6 +14,7 @@ #ifdef CONFIG_ARCH_RANDOM #include <linux/static_key.h> +#include <linux/preempt.h> #include <linux/atomic.h> #include <asm/cpacf.h> @@ -32,7 +33,8 @@ static inline bool __must_check arch_get static inline bool __must_check arch_get_random_seed_long(unsigned long *v) { - if (static_branch_likely(&s390_arch_random_available)) { + if (static_branch_likely(&s390_arch_random_available) && + in_task()) { cpacf_trng(NULL, 0, (u8 *)v, sizeof(*v)); atomic64_add(sizeof(*v), &s390_arch_random_counter); return true; @@ -42,7 +44,8 @@ static inline bool __must_check arch_get static inline bool __must_check arch_get_random_seed_int(unsigned int *v) { - if (static_branch_likely(&s390_arch_random_available)) { + if (static_branch_likely(&s390_arch_random_available) && + in_task()) { cpacf_trng(NULL, 0, (u8 *)v, sizeof(*v)); atomic64_add(sizeof(*v), &s390_arch_random_counter); return true; Patches currently in stable-queue which might be from freude@xxxxxxxxxxxxx are queue-5.18/s390-archrandom-prevent-cpacf-trng-invocations-in-interrupt-context.patch