bpf: Verifier, do_refine_retval_range may clamp umin to 0 incorrectly

do_refine_retval_range() is called to refine return values from specified
helpers, probe_read_str and get_stack at the moment, the reasoning is
because both have a max value as part of their input arguments and
because the helper ensure the return value will not be larger than this
we can set smax values of the return register, r0.

However, the return value is a signed integer so setting umax is incorrect
It leads to further confusion when the do_refine_retval_range() then calls,
__reg_deduce_bounds() which will see a umax value as meaning the value is
unsigned and then assuming it is unsigned set the smin = umin which in this
case results in 'smin = 0' and an 'smax = X' where X is the input argument
from the helper call.

Here are the comments from _reg_deduce_bounds() on why this would be safe
to do.

 /* Learn sign from unsigned bounds.  Signed bounds cross the sign
  * boundary, so we must be careful.
  */
 if ((s64)reg->umax_value >= 0) {
	/* Positive.  We can't learn anything from the smin, but smax
	 * is positive, hence safe.
	 */
	reg->smin_value = reg->umin_value;
	reg->smax_value = reg->umax_value = min_t(u64, reg->smax_value,
						  reg->umax_value);

But now we incorrectly have a return value with type int with the
signed bounds (0,X). Suppose the return value is negative, which is
possible the we have the verifier and reality out of sync. Among other
things this may result in any error handling code being falsely detected
as dead-code and removed. For instance the example below shows using
bpf_probe_read_str() causes the error path to be identified as dead
code and removed.

>From the 'llvm-object -S' dump,

 r2 = 100
 call 45
 if r0 s< 0 goto +4
 r4 = *(u32 *)(r7 + 0)

But from dump xlate

  (b7) r2 = 100
  (85) call bpf_probe_read_compat_str#-96768
  (61) r4 = *(u32 *)(r7 +0)  <-- dropped if goto

Due to verifier state after call being

 R0=inv(id=0,umax_value=100,var_off=(0x0; 0x7f))

To fix omit setting the umax value because its not safe. The only
actual bounds we know is the smax. This results in the correct bounds
(SMIN, X) where X is the max length from the helper. After this the
new verifier state looks like the following after call 45.

R0=inv(id=0,smax_value=100)

Then xlated version no longer removed dead code giving the expected
result,

  (b7) r2 = 100
  (85) call bpf_probe_read_compat_str#-96768
  (c5) if r0 s< 0x0 goto pc+4
  (61) r4 = *(u32 *)(r7 +0)

Note, bpf_probe_read_* calls are root only so we wont hit this case
with non-root bpf users.

v3: comment had some documentation about meta set to null case which
is not relevant here and confusing to include in the comment.

v2 note: In original version we set msize_smax_value from check_func_arg()
and propagated this into smax of retval. The logic was smax is the bound
on the retval we set and because the type in the helper is ARG_CONST_SIZE
we know that the reg is a positive tnum_const() so umax=smax. Alexei
pointed out though this is a bit odd to read because the register in
check_func_arg() has a C type of u32 and the umax bound would be the
normally relavent bound here. Pulling in extra knowledge about future
checks makes reading the code a bit tricky. Further having a signed
meta data that can only ever be positive is also a bit odd. So dropped
the msize_smax_value metadata and made it a u64 msize_max_value to
indicate its unsigned. And additionally save bound from umax value in
check_arg_funcs which is the same as smax due to as noted above tnumx_cont
and negative check but reads better. By my analysis nothing functionally
changes in v2 but it does get easier to read so that is win.

Fixes: 849fa50662fbc ("bpf/verifier: refine retval R0 state for bpf_get_stack helper")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/158560417900.10843.14351995140624628941.stgit@john-Precision-5820-Tower

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