Re: [PATCH] bpf: 32-bit RSH verification must truncate input before the ALU op

[Daniel Borkmann]

On 10/05/2018 06:17 PM, Jann Horn wrote:
> When I wrote commit 468f6eafa6c4 ("bpf: fix 32-bit ALU op verification"), I
> assumed that, in order to emulate 64-bit arithmetic with 32-bit logic, it
> is sufficient to just truncate the output to 32 bits; and so I just moved
> the register size coercion that used to be at the start of the function to
> the end of the function.
> 
> That assumption is true for almost every op, but not for 32-bit right
> shifts, because those can propagate information towards the least
> significant bit. Fix it by always truncating inputs for 32-bit ops to 32
> bits.
> 
> Also get rid of the coerce_reg_to_size() after the ALU op, since that has
> no effect.
> 
> Fixes: 468f6eafa6c4 ("bpf: fix 32-bit ALU op verification")
> Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>

Applied to bpf, thanks Jann!