The commit 3e5903eb9cff70730 ("vsprintf: Prevent crash when dereferencing invalid pointers") broke boot on several architectures. The common pattern is that probe_kernel_read() is not working during early boot because userspace access framework is not ready.
The check is only the best effort. Let's not rush with it during the early boot. Details: 1. Report on Power: Kernel crashes very early during boot with with CONFIG_PPC_KUAP and CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG The problem is the combination of some new code called via printk(), check_pointer() which calls probe_kernel_read(). That then calls allow_user_access() (PPC_KUAP) and that uses mmu_has_feature() too early (before we've patched features). With the JUMP_LABEL debug enabled that causes us to call printk() & dump_stack() and we end up recursing and overflowing the stack. Because it happens so early you don't get any output, just an apparently dead system. The stack trace (which you don't see) is something like: ... dump_stack+0xdc probe_kernel_read+0x1a4 check_pointer+0x58 string+0x3c vsnprintf+0x1bc vscnprintf+0x20 printk_safe_log_store+0x7c printk+0x40 dump_stack_print_info+0xbc dump_stack+0x8 probe_kernel_read+0x1a4 probe_kernel_read+0x19c check_pointer+0x58 string+0x3c vsnprintf+0x1bc vscnprintf+0x20 vprintk_store+0x6c vprintk_emit+0xec vprintk_func+0xd4 printk+0x40 cpufeatures_process_feature+0xc8 scan_cpufeatures_subnodes+0x380 of_scan_flat_dt_subnodes+0xb4 dt_cpu_ftrs_scan_callback+0x158 of_scan_flat_dt+0xf0 dt_cpu_ftrs_scan+0x3c early_init_devtree+0x360 early_setup+0x9c 2. Report on s390: vsnprintf invocations, are broken on s390. For example, the early boot output now looks like this where the first (efault) should be the linux_banner: [ 0.099985] (efault) [ 0.099985] setup: Linux is running as a z/VM guest operating system in 64-bit mode [ 0.100066] setup: The maximum memory size is 8192MB [ 0.100070] cma: Reserved 4 MiB at (efault) [ 0.100100] numa: NUMA mode: (efault) The reason for this, is that the code assumes that probe_kernel_address() works very early. This however is not true on at least s390. Uaccess on KERNEL_DS works only after page tables have been setup on s390, which happens with setup_arch()->paging_init(). Any probe_kernel_address() invocation before that will return -EFAULT. Fixes: 3e5903eb9cff70730 ("vsprintf: Prevent crash when dereferencing invalid pointers") Signed-off-by: Petr Mladek <pmla...@suse.com> --- lib/vsprintf.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/lib/vsprintf.c b/lib/vsprintf.c index 7b0a6140bfad..8b43a883be6b 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -640,8 +640,13 @@ static const char *check_pointer_msg(const void *ptr) if (!ptr) return "(null)"; - if (probe_kernel_address(ptr, byte)) - return "(efault)"; + /* User space address handling is not ready during early boot. */ + if (system_state <= SYSTEM_BOOTING) { + if ((unsigned long)ptr < PAGE_SIZE) + return "(efault)"; + } else { + if (probe_kernel_address(ptr, byte)) + return "(efault)"; return NULL; } -- 2.16.4