cfFilterPWGToRaster divides by zero for zero input resolution

Summary

cfFilterPWGToRaster() accepts a PWG Raster page header with a zero HWResolution component, then performs modulo/division using it while calculating output scaling.

Reproduction

Build current upstream with ASan/UBSan. Start from a one-page PWG Raster stream (2SaR / PwgRaster sync), retain a parseable header and one byte of gray pixel data, then set the input page HWResolution to [0, 76800]. The tested page is 1 x 1, 8-bit gray; the PPD requests a normal 300-dpi output page.

ASAN_OPTIONS=detect_leaks=0 PPD=candidate.ppd CONTENT_TYPE=application/vnd.cups-pwg 
  pwgtoraster 1 fuzzer fuzzer 1 'PageSize=Small MediaType=Plain Duplex=None' document.pwg >/dev/null

Result

Reproduced through the real pwgtoraster CLI with an ASan/UBSan build from libcupsfilters HEAD d63939da08eaef622a2443b7c9638d22c4d9831d and libppd HEAD 522af8dd135f4dde66b1aac8b9d067808bbe122d on 2026-07-19. ASan/UBSan reports an FPE in cupsfilters/pwgtoraster.c:1620:

if (doc->outheader.HWResolution[i] % doc->inheader.HWResolution[i])

Cause and expected behavior

Reject zero input or output resolution before modulo or division. The malformed page header is accepted before out_page() uses the unchecked component.

Start at cupsfilters/pwgtoraster.c:1620 and trace how the PWG page header reaches out_page() before the modulo and division. Use the provided pwgtoraster command with the ASan/UBSan build and zero HWResolution input; done means zero input or output resolution is rejected before arithmetic and the reproduction no longer reports an FPE.