[PATCH] init: fix allocated page overlapping with PTR_ERR

[Joel Granados]

On Thu, Apr 18, 2024 at 12:29:43PM +0200, Nam Cao wrote:
> There is nothing preventing kernel memory allocators from allocating a
> page that overlaps with PTR_ERR(), except for architecture-specific
> code that setup memblock.
> 
> It was discovered that RISCV architecture doesn't setup memblock
> corectly, leading to a page overlapping with PTR_ERR() being allocated,
> and subsequently crashing the kernel (link in Close: )
> 
> The reported crash has nothing to do with PTR_ERR(): the last page
> (at address 0xfffff000) being allocated leads to an unexpected
> arithmetic overflow in ext4; but still, this page shouldn't be
> allocated in the first place.
> 
> Because PTR_ERR() is an architecture-independent thing, we shouldn't
> ask every single architecture to set this up. There may be other
> architectures beside RISCV that have the same problem.
> 
> Fix this one and for all by reserving the physical memory page that
> may be mapped to the last virtual memory page as part of low memory.
> 
> Unfortunately, this means if there is actual memory at this reserved
> location, that memory will become inaccessible. However, if this page
> is not reserved, it can only be accessed as high memory, so this
> doesn't matter if high memory is not supported. Even if high memory is
> supported, it is still only one page.
> 
> Closes: https://lore.kernel.org/linux-riscv/878r1ibpdn.fsf@all.your.base.are.belong.to.us
> Signed-off-by: Nam Cao <namcao at linutronix.de>
> Cc: <stable at vger.kernel.org> # all versions
> ---
>  init/main.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/init/main.c b/init/main.c
> index 881f6230ee59..f8d2793c4641 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -900,6 +900,7 @@ void start_kernel(void)
>  	page_address_init();
>  	pr_notice("%s", linux_banner);
>  	early_security_init();
> +	memblock_reserve(__pa(-PAGE_SIZE), PAGE_SIZE); /* reserve last page for ERR_PTR */
>  	setup_arch(&command_line);
>  	setup_boot_config();
>  	setup_command_line(command_line);
> -- 
> 2.39.2
> 

I received a similar(ish) report recently
https://lore.kernel.org/oe-kbuild-all/202404211031.J6l2AfJk-lkp@intel.com/
regarding RISC-V in init/mail.c. Here is the meat of the report in case
you want to avoid going to the actual link:
"
...
   riscv64-linux-ld: section .data LMA [000000000099b000,0000000001424de7] overlaps section .text LMA [0000000000104040,000000000213c543]
   riscv64-linux-ld: section .data..percpu LMA [00000000024e2000,00000000026b46e7] overlaps section .rodata LMA [000000000213c580,000000000292d0dd]
   riscv64-linux-ld: section .rodata VMA [ffffffff8213c580,ffffffff8292d0dd] overlaps section .data VMA [ffffffff82000000,ffffffff82a89de7]
   init/main.o: in function `rdinit_setup':
>> init/main.c:613:(.init.text+0x358): relocation truncated to fit: R_RISCV_GPREL_I against symbol `__setup_start' defined in .init.rodata section in .tmp_vmlinux.kallsyms1
   net/ipv4/ipconfig.o: in function `ic_dhcp_init_options':
   net/ipv4/ipconfig.c:682:(.init.text+0x9b4): relocation truncated to fit: R_RISCV_GPREL_I against `ic_bootp_cookie'
   net/sunrpc/auth_gss/gss_krb5_mech.o: in function `gss_krb5_prepare_enctype_priority_list':
>> net/sunrpc/auth_gss/gss_krb5_mech.c:213:(.text.gss_krb5_prepare_enctype_priority_list+0x9c): relocation truncated to fit: R_RISCV_GPREL_I against `gss_krb5_enctypes.0'
   lib/maple_tree.o: in function `mas_leaf_max_gap':
>> lib/maple_tree.c:1512:(.text.mas_leaf_max_gap+0x2b8): relocation truncated to fit: R_RISCV_GPREL_I against `mt_pivots'
   lib/maple_tree.o: in function `ma_dead_node':
>> lib/maple_tree.c:560:(.text.mas_data_end+0x110): relocation truncated to fit: R_RISCV_GPREL_I against `mt_pivots'
   lib/maple_tree.o: in function `mas_extend_spanning_null':
>> lib/maple_tree.c:3662:(.text.mas_extend_spanning_null+0x69c): relocation truncated to fit: R_RISCV_GPREL_I against `mt_pivots'
   lib/maple_tree.o: in function `mas_mab_cp':
>> lib/maple_tree.c:1943:(.text.mas_mab_cp+0x248): relocation truncated to fit: R_RISCV_GPREL_I against `mt_pivots'
   lib/maple_tree.o: in function `mab_mas_cp':
>> lib/maple_tree.c:2000:(.text.mab_mas_cp+0x15c): relocation truncated to fit: R_RISCV_GPREL_I against `mt_pivots'
   lib/maple_tree.o: in function `mas_reuse_node':
>> lib/maple_tree.c:3416:(.text.mas_reuse_node+0x17c): relocation truncated to fit: R_RISCV_GPREL_I against `mt_slots'
   lib/maple_tree.o: in function `mt_free_walk':
>> lib/maple_tree.c:5238:(.text.mt_free_walk+0x15c): relocation truncated to fit: R_RISCV_GPREL_I against `mt_slots'
   lib/maple_tree.o: in function `mtree_lookup_walk':
   lib/maple_tree.c:3700:(.text.mtree_lookup_walk+0x94): additional relocation overflows omitted from the output
...

"

Could the fix that you have posted here be related to that report?
Comments are greatly appreciated.

Best
--

Joel Granados
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