[PATCH] vsprintf: Do not break early boot with probing addresses

Petr Mladek
2019-05-09T12:19:23Z

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 <pmladek@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