In the Linux kernel, the following vulnerability has been resolved: topology: Keep the cpumask unchanged when printing cpumap During fuzz

In the Linux kernel, the following vulnerability has been resolved:

topology: Keep the cpumask unchanged when printing cpumap

During fuzz testing, the following warning was discovered:

different return values (15 and 11) from vsnprintf("%*pbl
", ...)

test:keyward is WARNING in kvasprintf
WARNING: CPU: 55 PID: 1168477 at lib/kasprintf.c:30 kvasprintf+0x121/0x130
Call Trace:
kvasprintf+0x121/0x130
kasprintf+0xa6/0xe0
bitmap_print_to_buf+0x89/0x100
core_siblings_list_read+0x7e/0xb0
kernfs_file_read_iter+0x15b/0x270
new_sync_read+0x153/0x260
vfs_read+0x215/0x290
ksys_read+0xb9/0x160
do_syscall_64+0x56/0x100
entry_SYSCALL_64_after_hwframe+0x78/0xe2

The call trace shows that kvasprintf() reported this warning during the
printing of core_siblings_list. kvasprintf() has several steps:

(1) First, calculate the length of the resulting formatted string.

(2) Allocate a buffer based on the returned length.

(3) Then, perform the actual string formatting.

(4) Check whether the lengths of the formatted strings returned in
steps (1) and (2) are consistent.

If the core_cpumask is modified between steps (1) and (3), the lengths
obtained in these two steps may not match. Indeed our test includes cpu
hotplugging, which should modify core_cpumask while printing.

To fix this issue, cache the cpumask into a temporary variable before
calling cpumap_print_{list, cpumask}_to_buf(), to keep it unchanged
during the printing process.