From ab6f762f0f53162d41497708b33c9a3236d3609e Mon Sep 17 00:00:00 2001 From: Sergey Senozhatsky Date: Tue, 3 Mar 2020 20:30:02 +0900 Subject: printk: queue wake_up_klogd irq_work only if per-CPU areas are ready printk_deferred(), similarly to printk_safe/printk_nmi, does not immediately attempt to print a new message on the consoles, avoiding calls into non-reentrant kernel paths, e.g. scheduler or timekeeping, which potentially can deadlock the system. Those printk() flavors, instead, rely on per-CPU flush irq_work to print messages from safer contexts. For same reasons (recursive scheduler or timekeeping calls) printk() uses per-CPU irq_work in order to wake up user space syslog/kmsg readers. However, only printk_safe/printk_nmi do make sure that per-CPU areas have been initialised and that it's safe to modify per-CPU irq_work. This means that, for instance, should printk_deferred() be invoked "too early", that is before per-CPU areas are initialised, printk_deferred() will perform illegal per-CPU access. Lech Perczak [0] reports that after commit 1b710b1b10ef ("char/random: silence a lockdep splat with printk()") user-space syslog/kmsg readers are not able to read new kernel messages. The reason is printk_deferred() being called too early (as was pointed out by Petr and John). Fix printk_deferred() and do not queue per-CPU irq_work before per-CPU areas are initialized. Link: https://lore.kernel.org/lkml/aa0732c6-5c4e-8a8b-a1c1-75ebe3dca05b@camlintechnologies.com/ Reported-by: Lech Perczak Signed-off-by: Sergey Senozhatsky Tested-by: Jann Horn Reviewed-by: Petr Mladek Cc: Greg Kroah-Hartman Cc: Theodore Ts'o Cc: John Ogness Signed-off-by: Linus Torvalds --- kernel/printk/printk_safe.c | 11 +---------- 1 file changed, 1 insertion(+), 10 deletions(-) (limited to 'kernel/printk/printk_safe.c') diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c index b4045e782743..d9a659a686f3 100644 --- a/kernel/printk/printk_safe.c +++ b/kernel/printk/printk_safe.c @@ -27,7 +27,6 @@ * There are situations when we want to make sure that all buffers * were handled or when IRQs are blocked. */ -static int printk_safe_irq_ready __read_mostly; #define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) - \ sizeof(atomic_t) - \ @@ -51,7 +50,7 @@ static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq); /* Get flushed in a more safe context. */ static void queue_flush_work(struct printk_safe_seq_buf *s) { - if (printk_safe_irq_ready) + if (printk_percpu_data_ready()) irq_work_queue(&s->work); } @@ -402,14 +401,6 @@ void __init printk_safe_init(void) #endif } - /* - * In the highly unlikely event that a NMI were to trigger at - * this moment. Make sure IRQ work is set up before this - * variable is set. - */ - barrier(); - printk_safe_irq_ready = 1; - /* Flush pending messages that did not have scheduled IRQ works. */ printk_safe_flush(); } -- cgit v1.2.3-70-g09d2 From 8c4e93c362ff114def211d4629b120af86eb1275 Mon Sep 17 00:00:00 2001 From: Petr Mladek Date: Mon, 24 Feb 2020 13:13:31 +0100 Subject: printk: Prepare for nested printk_nmi_enter() There is plenty of space in the printk_context variable. Reserve one byte there for the NMI context to be on the safe side. It should never overflow. The BUG_ON(in_nmi() == NMI_MASK) in nmi_enter() will trigger much earlier. Signed-off-by: Petr Mladek Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Thomas Gleixner Reviewed-by: Alexandre Chartre Link: https://lkml.kernel.org/r/20200505134100.681374113@linutronix.de --- kernel/printk/internal.h | 8 +++++--- kernel/printk/printk_safe.c | 4 ++-- 2 files changed, 7 insertions(+), 5 deletions(-) (limited to 'kernel/printk/printk_safe.c') diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h index b2b0f526f249..660f9a6bf73a 100644 --- a/kernel/printk/internal.h +++ b/kernel/printk/internal.h @@ -6,9 +6,11 @@ #ifdef CONFIG_PRINTK -#define PRINTK_SAFE_CONTEXT_MASK 0x3fffffff -#define PRINTK_NMI_DIRECT_CONTEXT_MASK 0x40000000 -#define PRINTK_NMI_CONTEXT_MASK 0x80000000 +#define PRINTK_SAFE_CONTEXT_MASK 0x007ffffff +#define PRINTK_NMI_DIRECT_CONTEXT_MASK 0x008000000 +#define PRINTK_NMI_CONTEXT_MASK 0xff0000000 + +#define PRINTK_NMI_CONTEXT_OFFSET 0x010000000 extern raw_spinlock_t logbuf_lock; diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c index d9a659a686f3..e8791f206417 100644 --- a/kernel/printk/printk_safe.c +++ b/kernel/printk/printk_safe.c @@ -295,12 +295,12 @@ static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args) void notrace printk_nmi_enter(void) { - this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK); + this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET); } void notrace printk_nmi_exit(void) { - this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK); + this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET); } /* -- cgit v1.2.3-70-g09d2 From b0f51883f551b900a04a80f49fb0886caf7e9a12 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Mon, 24 Feb 2020 22:25:03 +0100 Subject: printk: Disallow instrumenting print_nmi_enter() It happens early in nmi_enter(), no tracing, probing or other funnies allowed. Specifically as nmi_enter() will be used in do_debug(), which would cause recursive exceptions when kprobed. Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Thomas Gleixner Reviewed-by: Alexandre Chartre Link: https://lkml.kernel.org/r/20200505134101.139720912@linutronix.de --- kernel/printk/printk_safe.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) (limited to 'kernel/printk/printk_safe.c') diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c index e8791f206417..4242403316bb 100644 --- a/kernel/printk/printk_safe.c +++ b/kernel/printk/printk_safe.c @@ -10,6 +10,7 @@ #include #include #include +#include #include "internal.h" @@ -293,12 +294,12 @@ static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args) return printk_safe_log_store(s, fmt, args); } -void notrace printk_nmi_enter(void) +void noinstr printk_nmi_enter(void) { this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET); } -void notrace printk_nmi_exit(void) +void noinstr printk_nmi_exit(void) { this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET); } -- cgit v1.2.3-70-g09d2 From 2a9e5ded9543436620a7fbc9329ddcc32bf97bc7 Mon Sep 17 00:00:00 2001 From: Petr Mladek Date: Wed, 20 May 2020 12:22:33 +0200 Subject: printk/kdb: Redirect printk messages into kdb in any context kdb has to get messages on consoles even when the system is stopped. It uses kdb_printf() internally and calls console drivers on its own. It uses a hack to reuse an existing code. It sets "kdb_trap_printk" global variable to redirect even the normal printk() into the kdb_printf() variant. The variable "kdb_trap_printk" is checked in printk_default() and it is ignored when printk is redirected to printk_safe in NMI context. Solve this by moving the check into printk_func(). It is obvious that it is not fully safe. But it does not make things worse. The console drivers are already called in this context by db_printf() direct calls. Reported-by: Sumit Garg Tested-by: Sumit Garg Reviewed-by: Daniel Thompson Acked-by: Sergey Senozhatsky Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20200520102233.GC3464@linux-b0ei --- kernel/printk/printk.c | 14 +------------- kernel/printk/printk_safe.c | 7 +++++++ 2 files changed, 8 insertions(+), 13 deletions(-) (limited to 'kernel/printk/printk_safe.c') diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 9fdd6a42ad6a..2167bb528dd3 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -35,7 +35,6 @@ #include #include #include -#include #include #include #include @@ -2047,18 +2046,7 @@ EXPORT_SYMBOL(vprintk); int vprintk_default(const char *fmt, va_list args) { - int r; - -#ifdef CONFIG_KGDB_KDB - /* Allow to pass printk() to kdb but avoid a recursion. */ - if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0)) { - r = vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args); - return r; - } -#endif - r = vprintk_emit(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args); - - return r; + return vprintk_emit(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args); } EXPORT_SYMBOL_GPL(vprintk_default); diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c index d9a659a686f3..7ccb821d0bfe 100644 --- a/kernel/printk/printk_safe.c +++ b/kernel/printk/printk_safe.c @@ -6,6 +6,7 @@ #include #include #include +#include #include #include #include @@ -359,6 +360,12 @@ void __printk_safe_exit(void) __printf(1, 0) int vprintk_func(const char *fmt, va_list args) { +#ifdef CONFIG_KGDB_KDB + /* Allow to pass printk() to kdb but avoid a recursion. */ + if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0)) + return vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args); +#endif + /* * Try to use the main logbuf even in NMI. But avoid calling console * drivers that might have their own locks. -- cgit v1.2.3-70-g09d2