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Mon, 16 Feb 2026 00:55:04 -0800 (PST) Message-ID: <00493037-e77e-4631-8594-64ad4e46fa40@suse.com> Date: Mon, 16 Feb 2026 09:55:07 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Content-Language: en-US To: "xen-devel@lists.xenproject.org" Cc: Juergen Gross , Stefano Stabellini , Oleksandr Tyshchenko , Oleksandr Andrushchenko From: Jan Beulich Subject: [PATCH] Xen/gnttab: improve error handling in gnttab_dma_{alloc,free}_pages() Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @suse.com) X-ZM-MESSAGEID: 1771232128420154100 Content-Type: text/plain; charset="utf-8" Both decrease-reservation and populate-physmap can succeed partially. For the former this may be pretty unlikely, but clearly the latter can hit both out-of-memory conditions in the hypervisor or denial because of the allocation exceeding the domain's allowance (there's no interaction with the balloon driver here either). In gnttab_dma_free_pages() we simply can't give back to the system what hasn't been re-filled with backing memory. For gnttab_dma_alloc_pages() both parts of the overall region need dealing with differently. While no present caller of gnttab_dma_free_pages() checks its return value, also don't use -EFAULT there: It really is an out-of-memory condition. (In gnttab_dma_alloc_pages() -EFAULT doesn't look quite right either, but I couldn't think of a clearly better error code there.) Fixes: 9bdc7304f536 ("xen/grant-table: Allow allocating buffers suitable fo= r DMA") Signed-off-by: Jan Beulich --- I'm likely screwing things up, as I can't understand how this is intended to work: args->dev_bus_addr shouldn't really be in pseudo-physical address space, or else the address isn't suitable for handing to a device in order to DMA to/from it. Hence using __phys_to_pfn() on it to then hand the result to pfn_to_page() feels bogus. Was all of this perhaps only ever intended for (tested with) translated domains? With the uses of xenmem_reservation_va_mapping_*() only being masquerade? Furthermore the allocated buffer ought to have been contiguous in machine / DMA space. Yet the way it's re-populated upon freeing of the area doesn't guarantee that at all. All of what is done assumes dma_free_{coherent,wc,attr}() is capable of freeing piecemeal. I only checked dma_release_from_dev_coherent() to fulfill this requirement. If this can't be relied upon, please consider this submission as merely a bug report (for someone else to fix). --- a/drivers/xen/grant-table.c +++ b/drivers/xen/grant-table.c @@ -1095,6 +1095,28 @@ int gnttab_dma_alloc_pages(struct gnttab ret =3D xenmem_reservation_decrease(args->nr_pages, args->frames); if (ret !=3D args->nr_pages) { pr_debug("Failed to decrease reservation for DMA buffer\n"); + if (ret >=3D 0) { + /* Free the part where decrease didn't work. */ + size_t done =3D ret << PAGE_SHIFT; + + xenmem_reservation_va_mapping_update(args->nr_pages - ret, + &args->pages[ret], + &args->frames[ret]); + + if (args->coherent) + dma_free_coherent(args->dev, size - done, + args->vaddr + done, + args->dev_bus_addr + done); + else + dma_free_wc(args->dev, size - done, + args->vaddr + done, + args->dev_bus_addr + done); + + if (!ret) /* Nothing else to do? */ + return -EFAULT; + + args->nr_pages =3D ret; + } ret =3D -EFAULT; goto fail; } @@ -1128,7 +1150,12 @@ int gnttab_dma_free_pages(struct gnttab_ ret =3D xenmem_reservation_increase(args->nr_pages, args->frames); if (ret !=3D args->nr_pages) { pr_debug("Failed to increase reservation for DMA buffer\n"); - ret =3D -EFAULT; + /* Can only free what has been re-filled. */ + if (!ret) + return -ENOMEM; + if (ret > 0) + args->nr_pages =3D ret; + ret =3D -ENOMEM; } else { ret =3D 0; }