Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> writes: > On 5 January 2017 at 12:51, Nicolai Stange <nicstange@xxxxxxxxx> wrote: >> Before invoking the arch specific handler, efi_mem_reserve() reserves >> the given memory region through memblock. >> >> efi_mem_reserve() can get called after mm_init() though -- through >> efi_bgrt_init(), for example. After mm_init(), memblock is dead and should >> not be used anymore. >> >> Let efi_mem_reserve() check whether memblock is dead and not do the >> reservation if so. Emit a warning from the generic efi_arch mem_reserve() >> in this case: if the architecture doesn't provide any other means of >> registering the region as reserved, the operation would be a nop. >> >> Fixes: 4bc9f92e64c8 ("x86/efi-bgrt: Use efi_mem_reserve() to avoid copying image data") >> Signed-off-by: Nicolai Stange <nicstange@xxxxxxxxx> >> --- >> Applicable to next-20170105. >> No changes to v2. >> Boot-tested on x86_64. >> >> drivers/firmware/efi/efi.c | 7 +++++-- >> 1 file changed, 5 insertions(+), 2 deletions(-) >> >> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c >> index 92914801e388..158a8df2f4af 100644 >> --- a/drivers/firmware/efi/efi.c >> +++ b/drivers/firmware/efi/efi.c >> @@ -403,7 +403,10 @@ u64 __init efi_mem_desc_end(efi_memory_desc_t *md) >> return end; >> } >> >> -void __init __weak efi_arch_mem_reserve(phys_addr_t addr, u64 size) {} >> +void __init __weak efi_arch_mem_reserve(phys_addr_t addr, u64 size) >> +{ >> + WARN(slab_is_available(), "efi_mem_reserve() has no effect"); >> +} >> >> /** >> * efi_mem_reserve - Reserve an EFI memory region >> @@ -419,7 +422,7 @@ void __init __weak efi_arch_mem_reserve(phys_addr_t addr, u64 size) {} >> */ >> void __init efi_mem_reserve(phys_addr_t addr, u64 size) >> { >> - if (!memblock_is_region_reserved(addr, size)) >> + if (!slab_is_available() && !memblock_is_region_reserved(addr, size)) >> memblock_reserve(addr, size); >> More context: /* * Some architectures (x86) reserve all boot services ranges * until efi_free_boot_services() because of buggy firmware * implementations. This means the above memblock_reserve() is * superfluous on x86 and instead what it needs to do is * ensure the @start, @size is not freed. */ efi_arch_mem_reserve(addr, size); } > I share Dave's concern: on x86, this will silently ignore the > reservation if slab_is_available() returns true, AFAICS, x86 has got an efi_arch_mem_reserve() which doesn't ignore the reservation at any stage. The default implementation of efi_arch_mem_reserve() used on ARM is empty though ... > so we should at least warn here. ... and this patch adds a WARN() to the empty stub. > I don't think this patch solves any known issues, so I'd > rather defer this for now, and pick up the discussion when Matt is > back, I'm fine with either way and yes, no splat has been observed in the wild. Just to make it explicit: the issue addressed here is a potential use-after-free (both, read and write) on memblock.reserved.regions in case of CONFIG_ARCH_DISCARD_MEMBLOCK=y. It would certainly make sense to clarify the commit description in the next iteration... Thanks, Nicolai -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html