(no subject)

[Itay Iellin]

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Subject: fs/binfmt_elf: Integer Overflow vulnerability report
Reply-To: 
I'm sharing a report of an integer overflow vulnerability I found (in 
fs/binfmt_elf.c). I sent and discussed this vulnerability report with members
of security@kernel.org. I'm raising this for public discussion, with approval
from Greg (greg@kroah.com).

On Sun, Aug 01, 2021 at 04:30:30PM +0300, Itay Iellin wrote:
> In fs/binfmt_elf.c, line 1193, e_entry value can be overflowed. This
> potentially allows to create a fake entry point field for an ELF file.
> 
> The local variable e_entry is set to elf_ex->e_entry + load_bias.
> Given an ET_DYN ELF file, without a PT_INTERP program header, with an 
> elf_ex->e_entry field in the ELF header, which equals to
> 0xffffffffffffffff(in x86_64 for example), and a load_bias which is greater 
> than 0, e_entry(the local variable) overflows. This bypasses the check of 
> BAD_ADDR macro in line 1241.
> 
> It is possible to set a large enough NO-OP(NOP) sled, before the
> actual code, modify the elf_ex->e_entry field so that elf_ex->e_entry+load_bias
> will be in the range where the NO-OP sled is mapped(because the offset
> of the PT_LOAD program header of the text segment can be controlled). 
> This is practically a guess, because load_bias is randomized, the ELF file can
> be loaded a large amount of times until elf_ex->e_entry + load_bias 
> is in the range of the NO-OP sled.
> To conclude, this bug potentially allows the creation of a "fake" entry point
> field in the ELF file header. 
> 
> Suggested git diff:
> 
> Add a BAD_ADDR test to elf_ex->e_entry to prevent from using an
> overflowed elf_entry value.
> 
> Signed-off-by: Itay Iellin <ieitayie@gmail.com>
> ---
>  fs/binfmt_elf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 439ed81e755a..b59dcd5857db 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1238,7 +1238,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  		kfree(interp_elf_phdata);
>  	} else {
>  		elf_entry = e_entry;
> -		if (BAD_ADDR(elf_entry)) {
> +		if (BAD_ADDR(elf_entry) || BAD_ADDR(elf_ex->e_entry)) {
>  			retval = -EINVAL;
>  			goto out_free_dentry;
>  		}
> -- 
> 2.32.0
> 

I am not attaching the replies to my initial report from the discussion with
members of security@kernel.org, only when or if I will be given permission
from the repliers to do so.

Itay Iellin