CVE-2024-57917: topology: Keep the cpumask unchanged when printing cpumap

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 bitmapprinttobuf+0x89/0x100 coresiblingslistread+0x7e/0xb0 kernfsfilereaditer+0x15b/0x270 newsyncread+0x153/0x260 vfsread+0x215/0x290 ksysread+0xb9/0x160 dosyscall64+0x56/0x100 entrySYSCALL64afterhwframe+0x78/0xe2

The call trace shows that kvasprintf() reported this warning during the printing of coresiblingslist. 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 corecpumask 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 corecpumask while printing.

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