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FAILED: patch "[PATCH] iommu/amd/pgtbl: Fix possible race while increase page table" failed to apply to 5.15-stable tree

PATCH [email protected] 1 participant 1 message Sep 21, 2025

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The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 1e56310b40fd2e7e0b9493da9ff488af145bdd0c
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025092103-skilled-expand-23a8@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 1e56310b40fd2e7e0b9493da9ff488af145bdd0c Mon Sep 17 00:00:00 2001
From: Vasant Hegde <[email protected]>
Date: Sat, 13 Sep 2025 06:26:57 +0000
Subject: [PATCH] iommu/amd/pgtbl: Fix possible race while increase page table
level

The AMD IOMMU host page table implementation supports dynamic page table levels (up to 6 levels), starting with a 3-level configuration that expands based on IOVA address. The kernel maintains a root pointer and current page table level to enable proper page table walks in alloc_pte()/fetch_pte() operations.

The IOMMU IOVA allocator initially starts with 32-bit address and onces its exhuasted it switches to 64-bit address (max address is determined based on IOMMU and device DMA capability). To support larger IOVA, AMD IOMMU driver increases page table level.

But in unmap path (iommu_v1_unmap_pages()), fetch_pte() reads pgtable->[root/mode] without lock. So its possible that in exteme corner case, when increase_address_space() is updating pgtable->[root/mode], fetch_pte() reads wrong page table level (pgtable->mode). It does compare the value with level encoded in page table and returns NULL. This will result is iommu_unmap ops to fail and upper layer may retry/log WARN_ON.

CPU 0 CPU 1

------                                       ------
map pages                                    unmap pages
alloc_pte() -> increase_address_space()      iommu_v1_unmap_pages() -> fetch_pte()

pgtable->root = pte (new root value)
READ pgtable->[mode/root]
Reads new root, old mode
Updates mode (pgtable->mode += 1)

Since Page table level updates are infrequent and already synchronized with a spinlock, implement seqcount to enable lock-free read operations on the read path.

Fixes 754265bcab7 ("iommu/amd: Fix race in increase_address_space()")
Reported-by Alejandro Jimenez <[email protected]>
Cc [email protected]
Cc Joao Martins <[email protected]>
Cc Suravee Suthikulpanit <[email protected]>
Signed-off-by Vasant Hegde <[email protected]>
Signed-off-by Joerg Roedel <[email protected]>
drivers/iommu/amd/amd_iommu_types.h +1 -0
index 5219d7ddfdaa..95f63c5f6159 100644
@@ -555,6 +555,7 @@ struct gcr3_tbl_info {
 };
 
 struct amd_io_pgtable {
+	seqcount_t		seqcount;	/* Protects root/mode update */
 	struct io_pgtable	pgtbl;
 	int			mode;
 	u64			*root;
drivers/iommu/amd/io_pgtable.c +21 -4
index a91e71f981ef..70c2f5b1631b 100644
@@ -17,6 +17,7 @@
 #include <linux/slab.h>
 #include <linux/types.h>
 #include <linux/dma-mapping.h>
+#include <linux/seqlock.h>
 
 #include <asm/barrier.h>
 

@@ -130,8 +131,11 @@ static bool increase_address_space(struct amd_io_pgtable *pgtable,
 
 	*pte = PM_LEVEL_PDE(pgtable->mode, iommu_virt_to_phys(pgtable->root));
 
+	write_seqcount_begin(&pgtable->seqcount);
 	pgtable->root  = pte;
 	pgtable->mode += 1;
+	write_seqcount_end(&pgtable->seqcount);
+
 	amd_iommu_update_and_flush_device_table(domain);
 
 	pte = NULL;

@@ -153,6 +157,7 @@ static u64 *alloc_pte(struct amd_io_pgtable *pgtable,
 {
 	unsigned long last_addr = address + (page_size - 1);
 	struct io_pgtable_cfg *cfg = &pgtable->pgtbl.cfg;
+	unsigned int seqcount;
 	int level, end_lvl;
 	u64 *pte, *page;
 

@@ -170,8 +175,14 @@ static u64 *alloc_pte(struct amd_io_pgtable *pgtable,
 	}
 
 
-	level   = pgtable->mode - 1;
-	pte     = &pgtable->root[PM_LEVEL_INDEX(level, address)];
+	do {
+		seqcount = read_seqcount_begin(&pgtable->seqcount);
+
+		level   = pgtable->mode - 1;
+		pte     = &pgtable->root[PM_LEVEL_INDEX(level, address)];
+	} while (read_seqcount_retry(&pgtable->seqcount, seqcount));
+
+
 	address = PAGE_SIZE_ALIGN(address, page_size);
 	end_lvl = PAGE_SIZE_LEVEL(page_size);
 

@@ -249,6 +260,7 @@ static u64 *fetch_pte(struct amd_io_pgtable *pgtable,
 		      unsigned long *page_size)
 {
 	int level;
+	unsigned int seqcount;
 	u64 *pte;
 
 	*page_size = 0;

@@ -256,8 +268,12 @@ static u64 *fetch_pte(struct amd_io_pgtable *pgtable,
 	if (address > PM_LEVEL_SIZE(pgtable->mode))
 		return NULL;
 
-	level	   =  pgtable->mode - 1;
-	pte	   = &pgtable->root[PM_LEVEL_INDEX(level, address)];
+	do {
+		seqcount = read_seqcount_begin(&pgtable->seqcount);
+		level	   =  pgtable->mode - 1;
+		pte	   = &pgtable->root[PM_LEVEL_INDEX(level, address)];
+	} while (read_seqcount_retry(&pgtable->seqcount, seqcount));
+
 	*page_size =  PTE_LEVEL_PAGE_SIZE(level);
 
 	while (level > 0) {

@@ -541,6 +557,7 @@ static struct io_pgtable *v1_alloc_pgtable(struct io_pgtable_cfg *cfg, void *coo
 	if (!pgtable->root)
 		return NULL;
 	pgtable->mode = PAGE_MODE_3_LEVEL;
+	seqcount_init(&pgtable->seqcount);
 
 	cfg->pgsize_bitmap  = amd_iommu_pgsize_bitmap;
 	cfg->ias            = IOMMU_IN_ADDR_BIT_SIZE;