platform/x86: hp-bioscfg: fix heap OOB read in sk_store() and kek_store()

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

platform/x86: hp-bioscfg: fix heap OOB read in sk_store() and kek_store()

sk_store() and kek_store() strip a trailing newline from the sysfs write before allocating the key buffer:

length = count;
if (buf[length - 1] == '\n')
	length--;
bioscfg_drv.spm_data.signing_key = kmemdup(buf, length, GFP_KERNEL);

but then pass the original "count" (not "length") as the copy size to hp_wmi_perform_query(), which memcpy()s that many bytes out of the "length"-sized allocation, reading one byte past it whenever the write ends in a newline, the normal case for a shell "echo" into sysfs.

KASAN confirms this directly:

BUG: KASAN: slab-out-of-bounds in hp_wmi_perform_query+0x1e9/0x460 [hp_bioscfg] Read of size 28 at addr ffff88813c8e2b80 by task python3/16022 ... sk_store+0xa7/0x240 [hp_bioscfg] kernfs_fop_write_iter+0x3e1/0x5d0 ... The buggy address is located 0 bytes inside of allocated 27-byte region [ffff88813c8e2b80, ffff88813c8e2b9b)

Reproduced identically for kek_store, and at multiple write sizes (28, 57, 201 bytes), each time reading exactly one byte past a kmemdup() allocation one byte smaller than the write.

Fix by passing "length" instead of "count" to hp_wmi_perform_query() in both functions.