page_pool: Fix PP_MAGIC_MASK to avoid crashing on some 32-bit arches

Overview

In the Linux kernel, the following vulnerability has been resolved:

page_pool: Fix PP_MAGIC_MASK to avoid crashing on some 32-bit arches

Helge reported that the introduction of PP_MAGIC_MASK let to crashes on boot on his 32-bit parisc machine. The cause of this is the mask is set too wide, so the page_pool_page_is_pp() incurs false positives which crashes the machine.

Just disabling the check in page_pool_is_pp() will lead to the page_pool code itself malfunctioning; so instead of doing this, this patch changes the define for PP_DMA_INDEX_BITS to avoid mistaking arbitrary kernel pointers for page_pool-tagged pages.

The fix relies on the kernel pointers that alias with the pp_magic field always being above PAGE_OFFSET. With this assumption, we can use the lowest bit of the value of PAGE_OFFSET as the upper bound of the PP_DMA_INDEX_MASK, which should avoid the false positives.

Because we cannot rely on PAGE_OFFSET always being a compile-time constant, nor on it always being >0, we fall back to disabling the dma_index storage when there are not enough bits available. This leaves us in the situation we were in before the patch in the Fixes tag, but only on a subset of architecture configurations. This seems to be the best we can do until the transition to page types in complete for page_pool pages.

v2:

• Make sure there's at least 8 bits available and that the PAGE_OFFSET bit calculation doesn't wrap

Affected

• Linux >= 4f51fb0d257ff4d406ec27966902de075e3b118e < 15b8a5b4cdc16e9a8bb2a548e12a0fd92997605a
• Linux >= ee62ce7a1d909ccba0399680a03c2dee83bcae95 < f62934cea32c8f7b11b747975d69bf5afe4264cf
• Linux >= ee62ce7a1d909ccba0399680a03c2dee83bcae95 < 95920c2ed02bde551ab654e9749c2ca7bc3100e0
• Linux c30ae60f41f9edd6e1b5cad41cf28ce04dae39e4
• Linux >= 6.12.34 < 6.12.54
• Linux >= 6.15.3 < 6.16
• Linux 6.16

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.