efi: stmm: Fix incorrect buffer allocation method

[ Upstream commit c5e81e67 ]

The communication buffer allocated by setup_mm_hdr() is later on passed
to tee_shm_register_kernel_buf(). The latter expects those buffers to be
contiguous pages, but setup_mm_hdr() just uses kmalloc(). That can cause
various corruptions or BUGs, specifically since commit 9aec2fb0
("slab: allocate frozen pages"), though it was broken before as well.

Fix this by using alloc_pages_exact() instead of kmalloc().

Fixes: c44b6be6 ("efi: Add tee-based EFI variable driver")
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>