Re: [PATCH net-next v1] tcp: Correct signedness in skb remaining space calculation

["Jiayuan Chen" <jiayuan.chen@linux.dev>]

2025/7/2 23:34, "Eric Dumazet" <edumazet@google.com> 写到:



> 
> On Wed, Jul 2, 2025 at 8:28 AM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
> 
> > 
> > July 2, 2025 at 22:02, "Eric Dumazet" <edumazet@google.com> wrote:
> > 
> >  On Wed, Jul 2, 2025 at 6:59 AM Eric Dumazet <edumazet@google.com> wrote:
> > 
> >  >
> > 
> >  > On Wed, Jul 2, 2025 at 6:42 AM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
> > 
> >  >
> > 
> >  > July 2, 2025 at 19:00, "Jiayuan Chen" <jiayuan.chen@linux.dev> wrote:
> > 
> >  >
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> >  > >
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> >  >
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> >  > > The calculation for the remaining space, 'copy = size_goal - skb->len',
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> >  >
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> >  > >
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> >  >
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> >  > > was prone to an integer promotion bug that prevented copy from ever being
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> >  > > negative.
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> >  >
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> >  > >
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> >  >
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> >  > > The variable types involved are:
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> >  >
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> >  >
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> >  > > copy: ssize_t (long)
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> >  >
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> >  > >
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> >  >
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> >  > > size_goal: int
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> >  >
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> >  > >
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> >  >
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> >  > > skb->len: unsigned int
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> >  >
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> >  > >
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> >  >
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> >  > > Due to C's type promotion rules, the signed size_goal is converted to an
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> >  > > unsigned int to match skb->len before the subtraction. The result is an
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> >  > >
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> >  >
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> >  > > unsigned int.
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> >  >
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> >  > >
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> >  >
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> >  > > When this unsigned int result is then assigned to the s64 copy variable,
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> >  > > it is zero-extended, preserving its non-negative value. Consequently,
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> >  > > copy is always >= 0.
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> >  > >
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> >  >
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> >  > To better explain this problem, consider the following example:
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> >  >
> > 
> >  > '''
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> >  >
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> >  > #include <sys/types.h>
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> >  >
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> >  > #include <stdio.h>
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> >  >
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> >  > int size_goal = 536;
> > 
> >  >
> > 
> >  > unsigned int skblen = 1131;
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> >  >
> > 
> >  > void main() {
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> >  >
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> >  > ssize_t copy = 0;
> > 
> >  >
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> >  > copy = size_goal - skblen;
> > 
> >  >
> > 
> >  > printf("wrong: %zd\n", copy);
> > 
> >  >
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> >  > copy = size_goal - (ssize_t)skblen;
> > 
> >  >
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> >  > printf("correct: %zd\n", copy);
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> >  >
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> >  > return;
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> >  >
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> >  > }
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> >  >
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> >  > '''
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> >  >
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> >  > Output:
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> >  >
> > 
> >  > '''
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> >  >
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> >  > wrong: 4294966701
> > 
> >  >
> > 
> >  > correct: -595
> > 
> >  >
> > 
> >  > '''
> > 
> >  >
> > 
> >  > Can you explain how one skb could have more bytes (skb->len) than size_goal ?
> > 
> >  >
> > 
> >  > If we are under this condition, we already have a prior bug ?
> > 
> >  >
> > 
> >  > Please describe how you caught this issue.
> > 
> >  >
> > 
> >  Also, not sure why copy variable had to be changed from "int" to "ssize_t"
> > 
> >  A nicer patch (without a cast) would be to make it an "int" again/
> > 
> >  I encountered this issue because I had tcp_repair enabled, which uses
> > 
> >  tcp_init_tso_segs to reset the MSS.
> > 
> >  However, it seems that tcp_bound_to_half_wnd also dynamically adjusts
> > 
> >  the value to be smaller than the current size_goal.
> > 
> 
> Okay, and what was the end result ?
> 
> An skb has a limited amount of bytes that can be put into it
> 
> (MAX_SKB_FRAGS * 32K) , and I can't see what are the effects of having
> 

Hi Eric,

I'm working with a reproducer generated by syzkaller [1], and its core
logic is roughly as follows:

'''
setsockopt(fd, TCP_REPAIR, 1)
connect(fd);
setsockopt(fd, TCP_REPAIR, -1)

send(fd, small);
sendmmsg(fd, buffer_2G);
'''

First, because TCP_REPAIR is enabled, the send() operation leaves the skb
at the tail of the write_queue. Subsequently, sendmmsg is called to send
2GB of data.

Due to TCP_REPAIR, the size_goal is reduced, which can cause the copy
variable to become negative. However, because of integer promotion bug
mentioned in the previous email, this negative value is misinterpreted as
a large positive number. Ultimately, copy becomes a huge value, approaching
the int32 limit. This, in turn, causes sk->sk_forward_alloc to overflow,
which is the exact issue reported by syzkaller.

On a related note, even without using TCP_REPAIR, the tcp_bound_to_half_wnd()
function can also reduce size_goal on its own. Therefore, my understanding is
that under extreme conditions, we might still encounter an overflow in
sk->sk_forward_alloc.

So, I think we have good reason to change copy to an int.