[Bug analyzer/106284] New: False positives from -Wanalyzer-tainted-array-index with optimized conditionals

[Bug analyzer/106284] New: False positives from -Wanalyzer-tainted-array-index with optimized conditionals

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106284

            Bug ID: 106284
           Summary: False positives from -Wanalyzer-tainted-array-index
                    with optimized conditionals
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

Consider:

#define LOWER_LIMIT 5
#define UPPER_LIMIT 10

static int arr[UPPER_LIMIT];

static int
called_by_test_1 (int iarg)
{
  return arr[iarg]; /* { dg-warning "without bounds checking" } */
}

int __attribute__((tainted_args))
test_1 (unsigned long ularg)
{
  return called_by_test_1 (ularg);
}

static int
called_by_test_2 (int iarg)
{
  if (iarg < LOWER_LIMIT || iarg > UPPER_LIMIT)
    return 0;
  return arr[iarg]; /* { dg-bogus "bounds checking" "" { xfail *-*-* } } */
}

int __attribute__((tainted_args))
test_2 (unsigned long ularg)
{
  return called_by_test_2 (ularg);
}

int __attribute__((tainted_args))
test_3 (int iarg)
{
  if (iarg < LOWER_LIMIT || iarg > UPPER_LIMIT)
    return 0;
  return arr[iarg]; /* { dg-bogus "bounds checking" "" { xfail *-*-* } } */
}

int __attribute__((tainted_args))
test_4 (unsigned int uiarg)
{
  if (uiarg > UPPER_LIMIT)
    return 0;
  return arr[uiarg]; /* { dg-bogus "bounds checking" } */
}


This gives false positives from -Wanalyzer-tainted-array-index at -O1 and above
at the xfails.

See: https://godbolt.org/z/ahq7G6b4n

Affects trunk and gcc 12.1; would possibly also affect 11 (but that reproducer
uses  __attribute__((tainted_args)) which gcc 12 added).

Seen via false positive on Linux kernel in drivers/usb/class/usblp.c in
function ‘usblp_set_protocol’ handling usblp_ioctl on IOCNR_SET_PROTOCOL, which
has:

  | 1337 |         if (protocol < USBLP_FIRST_PROTOCOL || protocol >
USBLP_LAST_PROTOCOL)
  |      |            ~
  |      |            |
  |      |            (15) following ‘false’ branch...
  |......
  | 1341 |         if (usblp->intf->num_altsetting > 1) {
  |      |            ~~~~~~~~~~~~
  |      |            |     |
  |      |            |     (16) ...to here
  |      |            (17) following ‘true’ branch...
  | 1342 |                 alts = usblp->protocol[protocol].alt_setting;
  |      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  |      |                      |
  |      |                      (18) ...to here
  |      |                      (19) use of attacker-controlled value ‘arg’ in
array lookup without bounds checking

where "arg" is "protocol" (but from the caller frame), and is checked at (15).

[Bug analyzer/106284] False positives from -Wanalyzer-tainted-array-index with optimized conditionals

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106284

David Malcolm <dmalcolm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2022-07-14

--- Comment #1 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
I'm testing a fix for this.

[Bug analyzer/106284] False positives from -Wanalyzer-tainted-array-index with optimized conditionals

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106284

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by David Malcolm <dmalcolm@gcc.gnu.org>:

https://gcc.gnu.org/g:0a8edfbd37d399d1103d86e134ba0a92f8c873c3

commit r13-1713-g0a8edfbd37d399d1103d86e134ba0a92f8c873c3
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Fri Jul 15 11:28:34 2022 -0400

    analyzer: fix taint false positive on optimized range checks [PR106284]

    PR analyzer/106284 reports a false positive from
    -Wanalyzer-tainted-array-index seen on the Linux kernel
    with a version of my patches from:
      https://gcc.gnu.org/pipermail/gcc-patches/2021-November/584372.html
    in drivers/usb/class/usblp.c in function âusblp_set_protocolâ handling
    usblp_ioctl on IOCNR_SET_PROTOCOL, which has:

      | 1337 |         if (protocol < USBLP_FIRST_PROTOCOL || protocol >
USBLP_LAST_PROTOCOL)
      |      |            ~
      |      |            |
      |      |            (15) following âfalseâ branch...
      |......
      | 1341 |         if (usblp->intf->num_altsetting > 1) {
      |      |            ~~~~~~~~~~~~
      |      |            |     |
      |      |            |     (16) ...to here
      |      |            (17) following âtrueâ branch...
      | 1342 |                 alts = usblp->protocol[protocol].alt_setting;
      |      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |      |                      |
      |      |                      (18) ...to here
      |      |                      (19) use of attacker-controlled value
âargâ in array lookup without bounds checking

    where "arg" is "protocol" (albeit from the caller frame, the ioctl
    callback), and is clearly checked at (15).

    The root cause is that at -O1 and above fold-const's build_range-check
    can optimize range checks
      (c>=low) && (c<=high)
    into
      (c-low>=0) && (c-low<=high-low)
    and thus into a single check:
      (unsigned)(c - low) <= (unsigned)(high-low).

    I initially attempted to fix this by detecting such conditions in
    region_model::on_condition, and calling on_condition for both of the
    implied conditions.  This turned out not to work since the current
    sm_context framework doesn't support applying two conditions
    simultaneously: it led to a transition from the old state to has_lb,
    then a transition from the old state *again* to has_ub, thus leaving
    the new state as has_ub, rather than the stop state.

    Instead, this patch fixes things by special-casing it within
    taint_state_machine::on_condition.

    gcc/analyzer/ChangeLog:
            PR analyzer/106284
            * sm-taint.cc (taint_state_machine::on_condition): Handle range
            checks optimized by build_range_check.

    gcc/testsuite/ChangeLog:
            PR analyzer/106284
            * gcc.dg/analyzer/torture/taint-read-index-2.c: New test.

    Signed-off-by: David Malcolm <dmalcolm@redhat.com>

[Bug analyzer/106284] False positives from -Wanalyzer-tainted-array-index with optimized conditionals

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106284

David Malcolm <dmalcolm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Should be fixed on trunk for gcc 13 by the above patch.

[Bug analyzer/106284] False positives from -Wanalyzer-tainted-array-index with optimized conditionals

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106284

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by David Malcolm <dmalcolm@gcc.gnu.org>:

https://gcc.gnu.org/g:434d521d118fc7e7759b2b42bdddfa70caec637b

commit r13-1747-g434d521d118fc7e7759b2b42bdddfa70caec637b
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Tue Jul 19 09:53:39 2022 -0400

    analyzer: log out-edge description in exploded_graph::process_node

    I found this logging tweak very helpful when working on
    PR analyzer/106284.

    gcc/analyzer/ChangeLog:
            * engine.cc (exploded_graph::process_node): Show any description
            of the out-edge when logging it for consideration.

    Signed-off-by: David Malcolm <dmalcolm@redhat.com>